KR101721968B1 - HIERARCHICAL MATERIAL INCLUDING URCHIN SHAPED MnO2, METHODE FOR SYNTHESIS THE SAME, AND AIR ELECTRODE AND METAL-AIR BATTERY INCLUDING THE SAME - Google Patents

Abstract

The present invention relates to a material comprising ultrafine carbon fiber coated with reduced graphene oxide, and urchin-shaped MnO_2 nanowire positioned on a surface of the ultrafine carbon fiber coated with reduced graphene oxide, a method for producing the same, an air electrode of a metal-air battery including the material, and a metal-air battery including the air electrode.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a material having a stratified structure including sea urchin-shaped MnO2, a method for producing the same, a cathode and a metal-air battery including the same, SAME}

The present invention relates to a material having a hierarchical structure including sea urchin-shaped MnO ₂ , a method for producing the same, a cathode and a metal-air cell including the same.

Energy storage devices play an important role in energy-dependent societies. Lithium-ion batteries (LIBs), the most widely used energy storage devices, are widely used in portable electronic devices such as mobile phones, cameras and laptop computers and can also be used as energy storage devices for electric vehicles and hybrid electric vehicles. However, the capacity of a lithium-ion battery is 100 to 200 Whkg ^{- 1} , which is three to ten times lower than the capacity required in a gasoline-powered automobile. Lithium-air batteries (LABs), on the other hand, have a capacity of about 5200 Whkg- ¹ and can be used in high energy devices, but are not practical due to the low amount of lithium, poor cycle stability and high overvoltage. Therefore, it is necessary to develop a new type of energy storage device that is economical, portable and safe to replace lithium-based batteries.

A sodium - air battery is a new type of energy storage device that has a theoretical capacity of 1683 Whkg ^{- 1} and is economical and environmentally friendly. It also has a high ionic conductivity in aqueous environments, non-aqueous environments and solid electrolytes within sodium based systems. A typical sodium-air battery is composed of metallic sodium, which is a negative electrode, and a high-porosity electrode, which is an air electrode, and the negative electrode and the air electrode are separated by a separator and impregnated with an electrolyte.

According to previous reports, rechargeable batteries can be designed according to the selection and deposition of a cathode known as an electrocatalyst. The electrocatalyst consumes oxygen through an oxygen reduction reaction (ORR) while the battery is being discharged. On the other hand, when charging, oxygen is generated through an oxygen generating reaction (OER) at a high voltage. New metals such as Pt, Pd, Ru, Au and Ag have excellent catalytic activity but are difficult to use because of their low capacity and high cost. In recent years, research has focused on economical transition metal oxide catalysts such as MnO ₂ , Co ₃ O ₄ and Fe ₃ O ₄ . Of these, MnO ₂ is attractive as an electrocatalyst since it has excellent catalytic activity. In addition, the MnO ₂ can be optimized for designing different morphologies of metal-air batteries.

On the other hand, graphene and its derivatives are recognized as excellent catalysts due to their high electrical conductivity, fast charge transport ability and high specific surface area. Graphene is also used as an adjunct to other catalysts because of its high electrochemical stability.

The present invention provides an electrode and a metal-air battery excellent in rechargeability and electrochemical characteristics, hard, low-cost, and high-capacity, by providing a material excellent in charge transfer, diffusion of electrolyte, diffusion of oxygen and electron transfer, .

In one embodiment of the present invention, ultrafine carbon fiber coated with reduced oxidized graphene, and urchin-like MnO ₂ located on the surface of the reduced graphene-coated ultrafine carbon fiber A material comprising a nanowire is provided.

The sea urchin-like MnO ₂ The nanowire may be directly connected to the reduced oxidized graphene without a binder.

Specifically, the ultrafine carbon fiber coated with the reduced oxidized graphene is coated with urchin-shaped microspheres, and the urchin-shaped microspheres are coated with radially aligned MnO ₂ And may be composed of nanowires.

The average diameter of the microspheres may be between 1 탆 and 15 탆.

The MnO ₂ The average diameter of the cross-section of the nanowire may be between 5 nm and 50 nm.

The reduced oxidized graphene may be porous.

The sea urchin-like MnO ₂ The nanowires may be MnO ₂ (α- MnO ₂₎ nanowires on alpha.

The sea urchin-like MnO ₂ The nanowire may be one that forms a V-type channel.

The BET specific surface area of the material may be from 10 m ² / g to 30 m ² / g.

According to another embodiment of the present invention, there is provided a method of manufacturing a carbon nanotube structure, comprising the steps of: coating ultrafine carbon fiber with reduced oxidized graphene; and forming urchin-shaped MnO ₂ nanowires on the reduced oxidized graphene- The method comprising the steps of:

The step of coating the reduced graphene oxide graphene on the ultrafine carbon fiber may include the steps of adding etchant particles to the graphene oxide dispersion, adding ultrafine carbon fiber to the mixed solution to heat treatment, and etching the etchant particles . ≪ / RTI >

The etchant particles may comprise polystyrene, silica, or a combination thereof.

The heat treatment may be performed at 100 캜 to 200 캜 for 6 hours to 24 hours.

The step of etching the etchant particles comprises immersing the coated ultrafine carbon fibers in an etch solution, wherein the etchant solution is selected from the group consisting of toluene, acetone, benzene, ethylbenzene, And may include xylene, dichloromethane, chloroform, carbon tetrachloride, tetrahydrofuran, or a combination thereof.

On the ultrafine carbon fiber coated with the reduced graphene oxide, sea urchin-shaped MnO ₂ The step of forming the nanowire may be by a hydrothermal synthesis method.

On the ultrafine carbon fiber coated with the reduced graphene oxide, sea urchin-shaped MnO ₂ The step of forming the nanowire may include the step of immersing the reduced graphene-coated ultrafine carbon fiber in a solution containing KMnO ₄ , and a heat treatment step.

The heat treatment may be performed at 100 ° C to 200 ° C for 12 hours to 48 hours.

Another embodiment of the present invention provides an air electrode of a metal-air battery including the above material.

According to still another embodiment of the present invention, there is provided a method of manufacturing a lithium secondary battery, which comprises the above air electrode, an aqueous electrolyte in contact with the air electrode, a negative electrode including an alkali metal, a non-aqueous electrolyte contacting the negative electrode, and an electrolyte membrane disposed between the positive electrode and the negative electrode Metal-air battery.

The metal-air battery may be a secondary aqueous Na-O ₂ battery.

The material of the hierarchical structure including sea urchin MnO ₂ according to an embodiment can facilitate the transport of electric charge in the cell and improve the durability and electrochemical characteristics of the cell. The air electrode and the metal-air battery including the above material are excellent in rechargeability, hard, and excellent in electrochemical properties.

Figure 1 is a sea urchin on a reduced graphene oxide coated ultra fine carbon fiber-shaped MnO ₂ And schematically illustrating a method for manufacturing a material in which a nanowire is formed.
Figures 2 to 5 are scanning electron microscope (SEM) photographs of the materials produced in the examples.
FIGS. 6 and 7 are transmission electron microscope (TEM) photographs of the materials produced in Examples. FIG.
8 is MnO ₂ An elemental mapping picture of a nanowire.
9 is an X-ray diffraction (XRD) result for the material of the example.
10 is a Raman spectrum for the material of the example.
FIG. 11 is an X-ray photoelectron spectroscopic analysis (XPS) result for the material of the example.
FIG. 12 is an oxygen reduction polarization curve of a disk electrode according to a rotating ring-disk electrode analysis method for Examples and Comparative Examples, and FIG. 13 is an oxygen generation reaction polarization curve.
14 is a constant current charging / discharging profile of a battery to which a sodium-air battery and a Pt / C air electrode of the embodiment are applied.
FIG. 15 is a graph comparing the refilling performance of the MGC electrode of the embodiment with that of the ultrafine carbon fiber, carbon black, and ultrafine carbon fiber electrode coated with reduced oxidized graphene.
16 is a graph for evaluating a rate capability for a battery of the embodiment.
FIG. 17 is a simplified view of a battery performance mechanism. FIG.
FIG. 18 is a graph comparing the performance of a battery in which sea urchin-shaped MnO ₂ is applied and a case in which flake-shaped MnO ₂ is applied.
19 is a charge / discharge profile according to the cycle of the example battery.
20 is a graph showing the potential difference between charging and discharging of the example battery.
21 is a SEM photograph of the electrode of Example 20 after 20 cycles.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

In one embodiment of the present invention, there is provided a carbon microfiber coated with reduced graphene oxide (rGO), and a carbon microfiber coated on the surface of the reduced graphene oxide- Sea urchin-shaped MnO ₂ (I.e., Urchin shaped MnO ₂ nanowires on rGO coated carbon microfiber (hereinafter referred to as MGC)) containing nanowires.

Generally, a binder is used for bonding an active material and a current collector to an electrode of a battery. Such a binder interferes with the movement of the ions and the battery, causing electrode polarization, increasing the overvoltage, and reducing the efficiency of recharging. In addition, when the binder is added, the weight of the electrode is increased, and a process of mixing the materials in the manufacturing process is added.

On the other hand, the urchin-like MnO ₂ The nanowire may be grown directly on the reduced graphene-coated ultrafine carbon fiber, and the urchin-like MnO ₂ nanowire may be directly connected to the reduced graphene graphene without a separate binder. Accordingly, the MnO ₂ High electrical contact can be formed between the nanowire, reduced oxidized graphene, and ultrafine carbon fibers, which can lead to effective ion and electron transport. In addition, when used for an electrode of a battery, the material may contribute to the production of a rigid battery and excellent rechargeability and cycle stability of the battery. In addition, the material has a hierarchical structure, and it is possible to provide a passage so that the electrolyte and the air can be effectively diffused in the battery. The above material is an urchin-like MnO ₂ Due to the nanowires, they have large active sites and have high conductivity due to reduced oxidized graphene. The ultrafine carbon fiber is advantageous in that it is economical, lightweight, and has high porosity as compared with other current collectors (for example, nickel foam, copper foil, aluminum foil, titanium mesh, etc.).

In detail, the ultrafine carbon fiber coated with reduced oxidized grains is coated with urchin-shaped microspheres, and the urchin-shaped microspheres are radially aligned The MnO ₂ Nanowire. ≪ / RTI > FIGS. 2 to 5 are SEM photographs of the materials prepared in Examples to be described later, showing a hierarchical appearance of the materials. Fig. 2 shows a microfabric network of ultrafine carbon fibers, Fig. 3 shows sea urchin microspheres on microfibers, and Figs. 4 and 5 show MnO ₂ Show nanowires.

The average diameter of the microspheres may be 1 탆 to 15 탆, specifically 5 탆 to 15 탆, 1 탆 to 1 탆, or 5 탆 to 10 탆. In this case, the material can exhibit excellent performance when used as an electrode.

The MnO ₂ The average diameter of the cross section of the nanowire is in the range of 5 nm to 50 nm, specifically 10 nm to 50 nm, 15 nm to 50 nm, 20 nm to 50 nm, 5 nm to 45 nm, 5 nm to 40 nm, nm, 5 nm to 30 nm, or 5 nm to 25 nm, and the like. In this case, the material can exhibit excellent performance when used as an electrode.

The reduced graphene oxide grains coated on the ultrafine carbon fiber may be porous and have a defect. These reduced graphene graphenes play an important role in the growth of MnO ₂ nanowires.

The urchin-like MnO ₂ may be a highly crystalline 慣 -MnO ₂ arranged in a sea urchin shape and radially aligned. Due to the sea urchin geometry, the MnO ₂ nanowires can create a continuous V-type channel, which is fully exposed to the active site. The V-type channel can facilitate diffusion of electrolyte in the cell and smooth contact between the air electrode and the electrolyte, and promote migration of metal ions and electrons. Furthermore, the nanowire shape can reduce the distance between the metal ions to be diffused in the redox process and the electrode polarization. At the same time, the highly crystalline MnO ₂ nanowires are well exposed to the diffusion of O ₂ and the transfer of electrons, which can greatly assist the oxygen reduction process. As a result, a battery using such a material can exhibit excellent electrochemical performance.

On the other hand, BET specific surface area of the material is 10 m ² / g to 30 m ² / g, specifically 10 m ² / g to ^{25 m 2 / g, 15 m} 2 / g to ^{30 m 2 / g, 15 m} 2 / g to 25 m ² / g, or from 20 m ² / g to 25 m ² / g. In this case, the material can exhibit excellent performance when used as an electrode.

According to another embodiment of the present invention, there is provided a method of manufacturing a carbon nanotube structure, comprising the steps of: coating ultrafine carbon fiber with reduced oxidized graphene; and forming urchin-shaped MnO ₂ nanowires on the reduced oxidized graphene- The method comprising the steps of:

On the ultrafine carbon fiber coated with the oxidized graphene reduced by the above-mentioned method, sea urchin-like MnO ₂ A nanowire-formed material can be produced. The material can be used as an electrode of a battery. According to the manufacturing method, a separate binder is added and mixed. This is a completely different method from the conventional blade coating method or mechanical pressing method which is manufactured by making slurry.

The step of coating the reduced graphene graphene on the ultrafine carbon fiber as the first step may include a step of adding ultrafine carbon fiber to the graphene oxide graphene dispersion and heat-treating the graphene dispersion.

Specifically, the first step includes a step of adding etching (etching) particles to the graphene oxide dispersion, a step of adding ultrafine carbon fibers to the mixed solution to heat treatment, and the step of etching the etching particles .

The etched particles are etched to form a reduced oxide graphene on the surface of the ultrafine carbon fiber, which is defective. These reduced graphene graphenes play an important role in the growth of MnO ₂ nanowires on the graphene surface. The growth mechanism of the MnO ₂ nanowires will be described in detail in Examples to be described later.

The etchant particles can be polystyrene nanoparticles, silica particles, or a combination thereof.

The heat treatment may be performed at a temperature of 100 ° C to 200 ° C, specifically at 110 ° C or higher, 120 ° C or higher, 130 ° C or higher, 140 ° C or higher, 190 ° C or lower, or 180 ° C or lower . The heat treatment may be performed for 6 to 24 hours.

The method of manufacturing the material may further include a washing step and / or a drying step after the heat treatment step.

The step of etching the etching particles may include immersing the coated ultrafine carbon fibers in an etching solution. The etching solution may specifically include at least one selected from the group consisting of toluene, acetone, benzene, ethylbenzene, xylene, dichloromethane, chloroform, carbon tetrachloride, tetrachloride, tetrahydrofuran, or combinations thereof.

In the second step, on the reduced graphene-coated ultrafine carbon fiber, sea urchin-shaped MnO ₂ The step of forming the nanowire may be by a hydrothermal synthesis method. Specifically, the second step may include a step of immersing the reduced graphene-coated ultrafine carbon fiber in a solution containing KMnO ₄ , and a heat treatment step.

The heat treatment may be performed at a temperature of 100 ° C to 200 ° C, specifically at a temperature of 110 ° C or more, or 120 ° C or more, and at a temperature of 190 ° C or less, 180 ° C or less, 170 ° C or less, . The heat treatment may be performed for 12 to 48 hours. The heat treatment temperature and time were urchin-shaped MnO ₂ It plays an important role in the uniform formation of nanowires.

Another embodiment of the present invention provides an air electrode of a metal-air battery including the above material. The material can be used as the air electrode of a metal-air battery without additional binder or conductor. In this case, the air electrode has high electrical contact between MnO ₂ , graphene, and conductive carbon paper, and it can be hardened to improve the electrochemical performance of the battery and to ensure excellent rechargeability. Such robust, binder-free and high-performance electrodes can be used in low-cost, high-capacity energy storage devices such as supercapacitors, metal-ion batteries, metal-air batteries, and fuel cells.

According to still another embodiment of the present invention, there is provided a method of manufacturing a lithium secondary battery, which comprises the above air electrode, an aqueous electrolyte in contact with the air electrode, a negative electrode including an alkali metal, a non-aqueous electrolyte contacting the negative electrode, and an electrolyte membrane disposed between the positive electrode and the negative electrode Metal-air battery.

The metal-air battery may be an aqueous battery or a rechargeable secondary battery. For example, the metal-air battery may be a sodium-air battery, specifically, a secondary water-based sodium-air battery.

The metal-air battery according to an embodiment has excellent rechargeability and excellent electrochemical characteristics such as lifetime characteristics. The electricity is a low-cost, high-capacity practical energy storage device, and can be used in electric vehicles and hybrid electric vehicles.

[ Example ]

The urchin-like α-MnO ₂ on ultrafine carbon fiber coated with reduced oxidized graphene can be synthesized as follows. A schematic manufacturing process is shown in Fig.

(1) Step 1: Reduced Oxidized graphene  Coated ultrafine Carbon fiber  Produce

Graphene oxide (GO) was fabricated by the modified Hummers method. 0.05 g of GO was dispersed in 10 mL of water through ultrasound treatment, and then 500 의 of polystyrene solution (Polysciences Inc.) was added and sonication was continued for 1 hour. 2 mL of the solution was transferred to a Teflon coated stainless steel autoclave with piranha treated 2 x 2 micro carbon fibers (JNT20, JNTG Co., Ltd. Korea). The entire reaction mixture was sonicated for 30 minutes, transferred to a hot air oven and heat treated at 160 < 0 > C for 12 hours. After the reaction, the autoclave was naturally cooled to room temperature, and ultrafine carbon fibers were taken out. The carbon paper was washed with water and ethanol to remove impurities and dried at 60 DEG C for 4 hours in an oven. The microfine carbon fibers were immersed in a toluene solution and then dried again to etch polystyrene after drying.

(2) Step 2: Preparation of sea urchin α- MnO ₂ on reduced oxide graphene- coated ultrafine carbon fibers

1.5 mmol KMnO ₄ (Sigma Aldrich) was dissolved in 2.5 mmol of concentrated HCl (Daejung Chemicals, Korea) and 15 mL of distilled water. The mixture was vigorously stirred for several minutes until a purple solution formed. The ultrafine carbon fiber coated with the reduced graphene grains previously prepared was immersed in the solution, and the entire reaction mixture was transferred to a Teflon coated stainless steel autoclave and heat-treated at 90 ° C for 4 hours in a hot air oven. The oven temperature was then raised to 140 ° C and held for 24 hours. After the reaction, the autoclave was naturally cooled to room temperature, and ultrafine carbon fibers were taken out. The carbon paper was washed several times with water and ethanol to remove impurities and dried at 60 DEG C for 3 hours in an oven.

(3) Synthesis mechanism of materials

Specifically, the mechanism by which urchin-shaped MnO ₂ grows on the reduced graphene-coated ultrafine carbon fiber is a mechanism in which MnO ₂ nanowires grow radially after the bubble-template aggregate of MnO ₂ nanocrystals is formed into spheres . The reduced graphene graphene has unreduced portions on the surface and the unreduced functional groups on the reduced oxidized graphene sheet produce gases such as CO ₂ , CO, and H ₂ in the solution. The resulting gas bubbles are caught in the defective part of the graphene surface and the aggregate of MnO ₂ nanocrystals is induced as a result of minimizing the interfacial energy at the liquid / gas interface. This aggregation process occurs during the hydrothermal synthesis process in which KMnO ₄ is reduced to MnO ₂ nanocrystals. Then, the spherically gathered MnO ₂ nanocrystals grow into radial nanowires under characteristic conditions to become sea urchin-shaped MnO ₂ nanowires.

At the nanowire growth stage, the reaction time (about 24 hours in the example) plays an important role in the uniform formation of MnO ₂ nanowires. When reacted for 6 hours, a flake shape was formed. For 12 hours, mixed flake and nanowire was formed. When reacted for 48 hours, thicker nanowires were formed.

On the other hand, when ultrafine carbon fibers not coated with graphene were used, a randomly entangled MnO ₂ nanowire network was formed.

(4) Secondary water system Na-O ₂  Manufacture of batteries

An MGC electrode without any binder or conductor additive was used directly on the cathode of the Na-O ₂ cell assembly. To make the cathode compartment, metal sodium (Sigma Aldrich) was attached to Ni foam and NaCF ₃ SO ₃ (Sigma Aldrich) in tetraethylene glycol dimethyl ether (TEGDME) solvent was used as the anode liquid. In another compartment, the prepared MGC air electrode was attached to a Ti mesh as a collector and a 0.1 M NaOH solution was used as the cathode solution. The left side of the air electrode was exposed for the passage of air. These two compartments were separated by a solid electrolyte membrane NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ).

(5) Methods and criteria for evaluation of material properties and electrochemical properties

Powder X-ray diffraction (XRD) was performed on Cu K? Radiation (? = 0.154 nm) with Bruker D8 using raw material 15? Lt; / RTI > X-ray photoelectron spectrometer (XPS; K-alpha, Thermo Fisher, UK) and a confocal Raman microscope (alpha 300R, WITEC , Germany) were used. Field emission scanning electron microscopy (FE-SEM; Hitachi, S-4800) was performed at 10 kV operating voltage to observe the surface morphology of the sample. Samples for transmission electron microscopy (TEM) were prepared by cutting a small piece of the electrode and ultrasonicating it in ethanol, dropping a small droplet from the ethanol dispersion on a carbon coated Cu lattice (200 mesh) Respectively. TEM and selected area electron diffraction (SAED) were analyzed with a JEOL JEM 2100 high resolution transmission electron microscope using an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDX) and elemental maps were analyzed with an instrument attached to HR-TEM (INCA, Oxford Instruments). BET specific surface area was analyzed by nitrogen adsorption method at liquid nitrogen temperature using Micromeritics ASAP 2020 nitrogen adsorption mechanism.

The electrochemical evaluation of the cell was performed with a WonATech workstation (WBCS3000L).

[ Evaluation example ]

(One) MGC  Analysis of surface morphology of materials

2 to 5 are SEM photographs showing the hierarchical structure of the MGC electrode manufactured in the embodiment. Fig. 2 shows a microfabric network of ultrafine carbon fibers, Fig. 3 shows sea urchin microspheres on microfibers, and Figs. 4 and 5 show MnO ₂ Show nanowires. Ultrafine carbon fibers coated with reduced oxidized graphene are uniformly coated with urchin-shaped microspheres (diameter 5 탆 to 10 탆), and the microspheres are coated with radially aligned MnO ₂ And nanowires (20 nm to 25 nm in diameter). The MnO ₂ The nanowires are uniformly grown on the ultrafine carbon fibers coated with the reduced oxidized graphene.

FIGS. 6 and 7 are TEM photographs of the produced MGC, and MnO ₂ It shows that the nanowire has a high crystal structure. Figure 7 shows that the growth of MnO ₂ nanowires occurs along the (200) plane with a spacing of 0.49 nm.

8 is MnO ₂ An elemental mapping picture of the nanowire, showing the presence of Mn and O.

(2) MGC  Surface structure analysis of materials

An XRD experiment was conducted on the MGC material, and the results are shown in Fig. X-ray analysis shows that pure alpha-phase MnO ₂ is formed and all peaks shown in the figure correspond to the standard JCPDS value (JCPDS card No. 44-0141). The crystallization of the compound was well performed and the analytical result was determined by the determination deficit parameter a = b = 0.97847; It is clear that this is perfectly matched to a square structure with c = 0.28630 nm. No impurity peaks were observed except for the peak at 2 < [deg.] ≫ near 26 DEG which appeared to be due to the reduced oxidized graphene coating layer.

(3) MGC  Chemical structure analysis of materials

The chemical bonding state of MGC materials was investigated by Raman analysis and XPS. In FIG. 10, the MGC material was analyzed through Raman spectrum and Raman bands were observed at 570 cm ^-1 and 650 cm ^-1 . The Raman band at 570 cm ^-1 is due to the Mn-O stretching vibration at the basal plane of the MnO ₆ sheet and the Raman band at 650 cm ^-1 relates to the MO symmetric stretching vibration of the MnO ₆ group. D band and G band were not observed in the Raman spectrum of the MGC material, which means that the ultrafine carbon fiber is completely covered with the MnO ₂ nanowire.

The XPS analysis results shown in Figure 11 clearly show that Mn, O and C elements are present in the MGC material produced.

(4) MGC  Electrocatalytic performance evaluation of electrode

The dual function of the electrocatalyst to lower the activation energy of the oxygen reduction reaction (ORR) and the oxygen generation reaction (OER) in metal-air cells is very important. In order to investigate the electrocatalytic ORR and OER activity of the MGC electrode prepared in the example, a rotating ring-disk electrode with an Hg / HgO reference electrode at 1600 rpm in an O ₂ saturated 0.1 M NaOH electrolyte solution RRDE) and compared with Pt / C, rGO and flake - shaped MnO ₂ electrodes.

FIG. 12 shows the ORR polarization curve of the disk electrode, showing that the activity of the MGC electrode of the embodiment increased sharply with increasing the potential. MGC electrode from 0.8 V vs Hg / HgO potential is the ORR was Bo peak current density of 5.7 mAcm ^-2, which Pt / C ^electrode, the results of a comparable (6.5 mAcm ^2). MGC electrode showed higher starting potential and current density than rGO electrode (-3.9 mAcm ^-2 ) and MnO ₂ flake electrode (-4.5 mAcm ^{- 2} ). The higher catalytic activity of the MGC electrode compared to the MnO ₂ flake electrode is believed to be due to the fact that the MGC material has a wide active site due to the meteorological ellipsoidal shape.

Dual functional electrocatalysts must have good adsorption of electrolyte ions, good diffusion of electrolyte ions and air to the catalyst surface, and high ion and charge transport capability for efficient electrocatalytic activity. In the MGC electrode of the embodiment, the highly crystalline urchin MnO ₂ is evaluated to satisfy the essential conditions of such a bifunctional electrocatalyst.

13 is a graph showing the OER electrocatalytic activity of the MGC electrode. As shown in Fig. 13, it can be seen that the MGC electrode has excellent OER electrocatalytic activity (highest OER peak current density and lowest OER starting potential). This is thought to be due to the hierarchical sex configuration of the electrodes and rGO.

In conclusion, the MGC electrode has excellent ORR and OER electrocatalytic activity and is suitable for use as a dual functional electrocatalyst in metal-air cells. Such an MGC electrode can reduce electrode polarization phenomenon in a battery.

(5) Rechargeability  evaluation

FIG. 14 is a profile of constant current charging (OER process) and discharging (ORR process) at 15 mAg ^-1 for 25 hours in a cell to which a sodium-air battery and a Pt / C air electrode of the embodiment are applied. When the cell was charged using the MGC electrode, the charging pressure was 2.70 V and reached 3.59 V after 10 hours and there was no increase in potential. During the discharge, the potential reached 2.9V. The charging overvoltage ( _chg ) is ~ 0.5 V and the discharging overvoltage ( _dis ) is ~ 0.2 V as compared with the theoretical redox potential (3.11 V) of the battery. Therefore, the charging / discharging potential difference (overvoltage difference) is only 0.7 V and the round trip efficiency is equal to 81%. The electrochemical performance of the sodium-air cell of the MGC electrode application was similar to that of the commercial Pt / C electrode and was superior to that of the lithium-oxygen battery and the non-aqueous sodium-air battery.

FIG. 15 is a graph comparing the refilling performance of the MGC electrode of the embodiment with that of the ultrafine carbon fiber, carbon black, and ultrafine carbon fiber electrode coated with reduced oxidized graphene. It can be seen from FIG. 15 that the battery of the embodiment shows superior performance.

FIG. 16 is a graph for evaluating the rate capability through charging and discharging at various current densities from 10 mAg ^-1 to 20 mAg ^-1 for the battery of the embodiment. Referring to FIG. 16, the overvoltage difference slightly increased with increasing current density, which is due to the rapid diffusion of electrolyte ions, demonstrating that the MGC electrode has excellent rate characteristics.

(6) Analysis of battery mechanism

The superior rechargeability of sodium - air cells using MGC cathode is believed to be due to the excellent electrocatalytic activity of sea urchin MnO ₂ and MGC materials. FIG. 17 shows a cell performance mechanism that can explain the superior performance of the sea urchin-shaped MGC electrode.

During the discharge, the positive Na metal in the cathode is oxidized to Na ⁺ ions and moves to the anode through the (Na (s) Na ⁺ + e ^- ) electrolyte. The generated electrons move through the external circuit, Lt; / RTI > The generated electrons reduce oxygen through an oxygen reduction reaction (ORR) process, and the reduced oxygen meets sodium ions at the surface of the air electrode. The air electrode consists of high-crystalline MnO ₂ arranged in a radially-aligned shape. Due to the sea urchin shape, the MnO ₂ nanorods create a continuous V-type channel, which is completely exposed to the active site. The V-type channel of the air electrode facilitates diffusion of the electrolyte and smooth contact between the air electrode and the electrolyte and promotes the movement of Na ⁺ ions and electrons. As mentioned earlier, good electrocatalysts adsorb electrolyte ions well and spread air well over the electrode surface. The V-type channel can enhance the electrocatalytic activity of the MGC electrode and make the cell rechargeable. Furthermore, the nanowire shape narrows the distance to Na ⁺ ions that must be diffused in the redox process and reduces electrode polarization. At the same time, the high-crystalline MnO ₂ nanowires are well exposed to O ₂ diffusion and electron transport, which greatly facilitates the oxygen reduction (ORR) process. Therefore, the MgC electrode can exhibit excellent rechargeability by the urchin-shaped MnO ₂ . In order to verify this, the performance of the cell with the sea urchin-shaped MnO ₂ was compared with that of the case where the flake-like MnO ₂ was applied. As shown in FIG. 18, the sea urchin-shaped MnO ₂ was superior to the flake shape.

In addition, the advantages of alpha-phase MnO ₂ , reduced oxide graphene to enhance conductivity and hydrophilicity, the absence of a binder, and high specific surface area (22 m ² g ^-1 ) .

(7) Evaluation of battery life characteristics

Charging and discharging of the batteries of the examples were carried out up to 20 cycles, and FIG. 19 shows charging / discharging profiles according to each cycle. No significant drop in potential was observed during the continuous charge / discharge test for 1000 hours at 15 mAg ^-1 . In FIG. 20, the potential difference between charge and discharge of the MGC electrode is shown graphically, and only a minute potential difference increase is observed at 0.7 V to 0.84 V. Referring to FIG. 20, it can be seen that the charge-discharge efficiency after 20 cycles is 78% to 81%, which means that the water-based sodium-air battery using the urchin-shaped MnO ₂ air electrode of the embodiment is excellent in rechargeability.

The excellent recharging performance of the MGC electrode appears to be due to its rugged electrode design (urchin shape, support of reduced graphene grains, and binder-free properties) and excellent electrocatalytic activity for ORR and OER processes. As discussed above, the MGC electrode exhibits similar performance in ORR activity and superior performance in OER activity as compared to the Pt / C electrode, thereby leading to a highly repeatable redox process, Discharge repetition performance can be exhibited.

On the other hand, the robust nature of the MGC electrode also seems to contribute to excellent rechargeability. The robust nature of the MGC electrode can be demonstrated by comparing the SEM images before and after the electrochemical charge and discharge experiments of the MGC electrode. Fig. 21 is a SEM photograph of the MGC electrode after 20 cycles (at 15 mAg < ^-1 > for 1000 hours) showing almost no change in electrode shape. This means that the active material and the collector are strongly bonded.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. As will be understood by those skilled in the art. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (20)

Which is a material of a hirerarchical structure including carbon microfiber, reduced oxidized graphene (rGO), and sea urchin MnO ₂ nanowire,
The hirerarchical structure may be a < RTI ID = 0.0 >
Wherein the carbon microfiber is coated with the reduced oxidation graphene (rGO)
The ultrafine fibers coated with the reduced oxidized graphene (rGO) are coated with the urchin-shaped microspheres,
Wherein said urchin-like microspheres are comprised of radially aligned MnO ₂ nanowires.
material. The method of claim 1,
The sea urchin-like MnO ₂ Wherein the nanowire is directly connected to the reduced graphene graphene without a binder. delete The method of claim 1,
Wherein the average diameter of the microspheres is 1 占 퐉 to 15 占 퐉. The method of claim 1,
Wherein the average diameter of the cross-section of the MnO ₂ nanowire is 5 nm to 50 nm. The method of claim 1,
Wherein the reduced graphene graphene is porous. The method of claim 1,
The sea urchin-like MnO ₂ Nanowires are materials that are alpha-phase MnO ₂ (α-MnO ₂ ) nanowires. The method of claim 1,
The sea urchin-like MnO ₂ Wherein the nanowire forms a V-type channel. The method of claim 1,
Wherein the BET specific surface area of the material is from 10 m ² / g to 30 m ² / g. Coating the reduced grafted carbon fiber with reduced oxidized graphene, and
And forming urchin-like MnO ₂ nanowires on the ultrafine carbon fiber coated with reduced oxidized graphene,
The step of coating the reduced graphene oxide graphene on the ultrafine carbon fiber may include the steps of adding etchant particles to the graphene oxide dispersion, adding ultrafine carbon fiber to the prepared mixed solution and heat treating the etchant, , ≪ / RTI >
In the step of heat treatment by the addition of ultrafine carbon fiber in the prepared mixture solution, MnO ₂ bubbles of the nano-crystals - after the template assembly is formed in a spherical shape, the reduced oxidation graphene MnO ₂ on the coated ultra-fine carbon fiber nano Wherein the wire is radially grown,
Lt; / RTI > delete 11. The method of claim 10,
Wherein the etchant particles comprise polystyrene, silica, or a combination thereof. 11. The method of claim 10,
Wherein the heat treatment is performed at 100 占 폚 to 200 占 폚 for 6 hours to 24 hours. 11. The method of claim 10,
The step of etching the etchant particles comprises immersing the coated ultrafine carbon fibers in an etching solution,
The etching solution may be an organic solvent such as toluene, acetone, benzene, ethylbenzene, xylene, dichloromethane, chloroform, carbon tetrachloride, , Tetrahydrofuran, or a combination thereof. 11. The method of claim 10,
On the ultrafine carbon fiber coated with the reduced graphene oxide, sea urchin-shaped MnO ₂ Wherein the step of forming the nanowires is by a hydrothermal synthesis method. 11. The method of claim 10,
On the ultrafine carbon fiber coated with the reduced graphene oxide, sea urchin-shaped MnO ₂ The step of forming the nanowire
Immersing the ultra-fine carbon fiber coated with the reduced oxidized graphene in a solution containing KMnO ₄ , and a heat treatment step. 17. The method of claim 16,
Wherein the heat treatment is performed at 100 占 폚 to 200 占 폚 for 12 hours to 48 hours. An air electrode of a metal-air battery comprising a material according to any one of claims 1, 2 and 4 to 9. The air electrode of claim 18,
A water-based electrolyte in contact with the air electrode,
A negative electrode containing an alkali metal,
A non-aqueous electrolyte in contact with the negative electrode, and
And an electrolyte membrane positioned between the cathode and the cathode. 20. The method of claim 19,
The metal-air battery is a secondary aqueous Na-O ₂ battery.

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