CATALYST FOR OXYGEN REDUCTION REACTION AND OXYGEN EVOLUTION REACTION AND METHOD FOR MANUFACTURING OF THE SAME

Abstract

Disclosed is a catalyst for oxygen reduction and evolution reactions. The catalyst is in the form of nickel sulfide (NiS.sub.2) nanosheets. NiS.sub.2 molecules are cross-linked and oriented two-dimensionally in the NiS.sub.2 nanosheets. Also disclosed is a method for producing the catalyst.

Claims

1. A catalyst for oxygen reduction and evolution reactions in the form of nickel sulfide (NiS.sub.2) nanosheets wherein NiS.sub.2 molecules are cross-linked and oriented two-dimensionally.

2. The catalyst according to claim 1, wherein the catalyst is used for a cathode of a lithium-air battery.

3. The catalyst according to claim 1, wherein the NiS.sub.2 nanosheets have a single crystalline structure.

4. The catalyst according to claim 1, wherein the {200} crystal plane of the NiS.sub.2 nanosheets is exposed.

5. A method for producing a catalyst for oxygen reduction and evolution reactions, comprising synthesizing a nickel hydroxide (Ni(OH).sub.2) nanosheet precursor in which Ni(OH).sub.2 molecules are cross-linked and oriented two-dimensionally, and reacting the Ni(OH).sub.2 nanosheet precursor with sulfur to synthesize nickel sulfide (NiS.sub.2) nanosheets.

6. The method according to claim 5, wherein the synthesis of the Ni(OH).sub.2 nanosheet precursor comprises adding an ammonium solution to a solution of nickel acetate in deionized water to prepare a mixed solution and subjecting the mixed solution to hydrothermal synthesis.

7. The method according to claim 6, further comprising cleaning with a cleaning liquid and drying after the hydrothermal synthesis.

8. The method according to claim 6, wherein the ammonium solution is added in such an amount that the pH of the mixed solution is 9 to 11.

9. The method according to claim 6, wherein the hydrothermal synthesis is performed at 160 to 210 C.

10. The method according to claim 5, wherein the NiS.sub.2 nanosheets are synthesized by annealing the sulfur powder and reacting the resulting sulfur vapor with the Ni(OH).sub.2 nanosheet precursor.

11. The method according to claim 10, wherein the NiS.sub.2 nanosheets are synthesized by reaction of the Ni(OH).sub.2 nanosheet precursor and the sulfur powder in a molar ratio of 1:5-25.

12. The method according to claim 10, wherein the annealing is performed at 350 to 550 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a schematic diagram showing a method for producing a catalyst for oxygen reduction and evolution reactions according to the present invention;

[0026] FIGS. 2a to 2c are scanning electron microscopy (SEM) images of Ni(OH).sub.2 precursors prepared in Example 1;

[0027] FIGS. 3a to 3e are scanning electron microscopy (SEM) images of NiS.sub.2 catalysts prepared in Example 2;

[0028] FIG. 4 shows X-ray diffraction (XRD) patterns of a Ni(OH).sub.2 precursor prepared in Example 1 and a NiS.sub.2 catalyst produced in Example 2;

[0029] FIG. 5 shows scanning electron microscopy (SEM) images of a Ni(OH).sub.2 precursor prepared in Example 1 and a NiS.sub.2 catalyst produced in Example 2;

[0030] FIG. 6 shows X-ray photoelectron spectroscopy (XPS) spectra of a Ni(OH).sub.2 precursor prepared in Example 1 and a NiS.sub.2 catalyst produced in Example 2;

[0031] FIG. 7 shows transmission electron microscopy (TEM) images, high-resolution TEM images, and fast Fourier transform (FFT) patterns of a Ni(OH).sub.2 precursor prepared in Example 1 and a NiS.sub.2 catalyst produced in Example 2;

[0032] FIG. 8 shows the results of cyclic voltammetry for oxygen reduction/resolution reactions (ORR/OER) in lithium-air batteries fabricated in Example 3 and Comparative Example 1;

[0033] FIG. 9 compares the performance of a lithium-air battery fabricated in Example 3 with that of a lithium-air battery fabricated in Comparative Example 1;

[0034] FIG. 10 shows scanning electron microscopy (SEM) images of a NiS.sub.2 catalyst after discharging and charging of a lithium-air battery fabricated in Example 3;

[0035] FIG. 11 shows the cycle stability of a lithium-air battery fabricated in Example 4; and

[0036] FIG. 12 compares the life characteristics of a NiS.sub.2 catalyst produced in Example 2 with those of conventional transition metal sulfide compositions.

DETAILED DESCRIPTION OF THE INVENTION

[0037] Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description and preferred embodiments with reference to the appended drawings. In the drawings, the same elements are denoted by the same reference numerals even though they are depicted in different drawings. Although such terms as first and second, etc. may be used to describe various elements, these elements should not be limited by above terms. These terms are used only to distinguish one element from another. In the description of the present invention, detailed explanations of related art are omitted when it is deemed that they may unnecessarily obscure the essence of the present invention.

[0038] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0039] A catalyst for oxygen reduction and evolution reactions according to one embodiment of the present invention is in the form of nickel sulfide (NiS.sub.2) nanosheets wherein NiS.sub.2 molecules are cross-linked and oriented two-dimensionally.

[0040] The bifunctional catalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) can be provided for a cathode of a lithium-air battery. When a lithium-air battery is discharged, lithium metal is oxidized at the anode to produce lithium ions and electrons. The lithium ions and the electrons move to the cathode through the electrolyte and circuit, respectively. Oxygen is reduced by the electrons at the cathode to form Li.sub.2O.sub.2. When the battery is charged, reverse reactions occur. That is, the lithium compound is decomposed to evolve oxygen gas at the cathode and the lithium ions are reduced at the anode. Such lithium-air batteries are estimated to have higher energy density than other next generation secondary batteries subsequent to lithium ion secondary batteries. However, conventional lithium-air batteries have a problem in that it is substantially impossible to reduce the difference in overpotential between the oxygen reduction reaction and the oxygen evolution reaction, failing to achieve relatively high electroactivity. The use of expensive precious metals such as gold and platinum (Pt) in conventional catalysts for the purpose of reducing the overpotential between the oxygen reduction reaction and the oxygen evolution reaction limits the commercialization of the catalysts. The catalyst for oxygen reduction and evolution reactions according to the present invention was invented as a solution to the problems of conventional lithium-air batteries.

[0041] Specifically, the catalyst for oxygen reduction and evolution reactions according to the present invention is in the form of nickel sulfide (NiS.sub.2) nanosheets. In the NiS.sub.2 nanosheets, NiS.sub.2 molecules are cross-linked and oriented two-dimensionally. The NiS.sub.2-based catalyst has high catalytic activity and is inexpensive. The NiS.sub.2 catalyst is relatively highly active for the oxygen evolution reaction. Therefore, the NiS.sub.2 catalyst can be utilized as a bifunctional catalyst for both oxygen reduction and oxygen evolution reactions. The NiS.sub.2 catalyst has a two-dimensional structure of nanosheets through structural control. Due to this structure, the NiS.sub.2 nanosheet catalyst has a large catalytically active area.

[0042] In addition, a highly active, specific plane of the NiS.sub.2 nanosheet catalyst is exposed in a controlled manner, ensuring high activity of the catalyst for oxygen reduction and oxygen evolution reactions and long-term catalytic stability of the catalyst. The NiS.sub.2 nanosheet catalyst has a single crystalline structure and its {200} crystal plane may be exposed.

[0043] Overall, according to the present invention, the use of the NiS.sub.2 catalyst whose catalytically active area is increased through structural control and whose highly active, specific plane is exposed enables the fabrication of a lithium-air battery with high capacity and stability. That is, the NiS.sub.2 catalyst of the present invention achieves improved activity for reversible reduction/oxidation reactions between lithium ions and oxygen and enhances the cycle stability of a lithium-air battery. In addition, the novel NiS.sub.2 catalyst of the present invention is highly active and inexpensive compared to precious metal catalysts such as gold and platinum catalysts and is thus advantageous for commercialization. Furthermore, the NiS.sub.2 catalyst of the present invention can be produced in a simple manner by a method described hereinbelow. Therefore, the NiS.sub.2 catalyst of the present invention can be produced on a large scale at low cost and is expected to contribute to the commercialization of lithium-air batteries in the near future.

[0044] The NiS.sub.2 catalyst can be produced according to the following method. FIG. 1 is a schematic diagram showing a method for producing a catalyst for oxygen reduction and evolution reactions according to the present invention. As shown in FIG. 1, a method for producing a catalyst for oxygen reduction and evolution reactions according to one embodiment of the present invention includes synthesizing a nickel hydroxide (Ni(OH).sub.2) nanosheet precursor in which Ni(OH).sub.2 molecules are cross-linked and oriented two-dimensionally, and reacting the Ni(OH).sub.2 nanosheet precursor with sulfur to synthesize nickel sulfide (NiS.sub.2) nanosheets.

[0045] First, a Ni(OH).sub.2 nanosheet precursor is synthesized by the following procedure. Nickel acetate is dissolved in deionized water to prepare a nickel acetate solution and an ammonium solution is added to the nickel acetate solution to prepare a mixed solution. The ammonium solution is added in such an amount that the pH of the mixed solution is 9 to 11, preferably 9.0 to 9.3, more preferably 9.1 to 9.2.

[0046] Next, the mixed solution is subjected to hydrothermal synthesis. The hydrothermal synthesis may be performed at 160 to 210 C., preferably 170 to 200 C., for 3 to 13 hours, preferably 4 to 12 hours.

[0047] After completion of the hydrothermal synthesis, the reaction product is cleaned with a cleaning liquid such as water or ethanol and dried to synthesize a Ni(OH).sub.2 nanosheet precursor. The Ni(OH).sub.2 nanosheet precursor is in the form of nanosheets in which Ni(OH).sub.2 molecules are cross-linked and oriented two-dimensionally.

[0048] Nickel sulfide (NiS.sub.2) nanosheets can be synthesized from the Ni(OH).sub.2 nanosheet precursor by a solid/gas phase reaction.

[0049] The Ni(OH).sub.2 nanosheet precursor is allowed to react with sulfur (S) to synthesize NiS.sub.2 nanosheets in which nickel sulfide (NiS.sub.2) molecules are cross-linked and oriented two-dimensionally. Specifically, a sulfur powder is vaporized by annealing and the resulting sulfur vapor is allowed to react the Ni(OH).sub.2 nanosheet precursor. For example, the Ni(OH).sub.2 nanosheet precursor is placed at the center of a tube furnace, the sulfur powder is arranged upstream of the tube such that it is spaced a distance from the Ni(OH).sub.2 nanosheet precursor, and annealing is performed under a flow of argon (Ar) gas. Here, the distance between the Ni(OH).sub.2 nanosheet precursor and the sulfur powder may be adjusted to 5 to 12 cm. The molar ratio of the Ni(OH).sub.2 nanosheet precursor to the sulfur powder may be adjusted to 1:5-25. The annealing may be performed at 350 to 550 C. for 0.5 to 1.5 hours. In one embodiment, the annealing may be performed at a temperature of 400 C. for 1 hour and the molar ratio and distance between the Ni(OH).sub.2 nanosheet precursor and the sulfur may be adjusted to 1:20 and 7 cm, respectively. In an alternative embodiment, the annealing may be performed at a temperature of 500 C. for 1 hour and the molar ratio and distance between the Ni(OH).sub.2 nanosheet precursor and the sulfur may be adjusted to 1:20 and 10 cm, respectively.

[0050] The present invention will be explained in more detail with reference to the following examples, including evaluation examples.

Example 1: Synthesis of Ni(OH).SUB.2 .Precursors

[0051] 0.08 mol of nickel acetate was sufficiently dissolved in 100 ml of water as a solvent, and an ammonium solution was added thereto to prepare mixed solutions in the range of pH 9-12. Each of the mixed solution was subjected to hydrothermal synthesis in the temperature range of 140 C. to 200 C. for 4-12 h while raising the temperature by 10 C. at 1 h intervals. After completion of the reaction, the reaction product was cleaned with water and ethanol and dried to synthesize a Ni(OH).sub.2 precursor.

Example 2: Synthesis of NiS.SUB.2 .Catalysts

[0052] The Ni(OH).sub.2 precursor prepared by hydrothermal synthesis of the mixed solution of pH 9.17 at 170 C. for 12 h was placed at the center of a tube furnace and annealing was performed under a flow of 20 sccm argon (Ar) gas while adjusting the distance between a sulfur powder and from the precursor to 7-10 cm by 1 cm increments. Here, the molar ratio between the precursor and the sulfur powder was adjusted to 1:10 and 1:20 and the annealing was performed in the temperature range of 400500 C. for 1 h while raising the temperature by 50 C. Various NiS.sub.2 catalysts were produced under different synthesis conditions by varying the molar ratio and distance between the precursor and the sulfur powder and the annealing temperature.

Example 3: Fabrication of Lithium-Air Battery Using the NiS.SUB.2 .Catalyst

[0053] The NiS.sub.2 catalyst (45 wt %) produced in Example 2, Super P carbon black (45 wt %), and polytetrafluoroethylene (PTFE=10 wt %) were mixed and dispersed in ethanol as a solvent to prepare an electrode ink. 0.5 mg of the electrode ink was loaded into a nickel foam (diameter=1 cm) by dip coating and was then sufficiently dried. A cell composed of a cathode, an anode, a separator, and an electrolyte was prepared. The anode was a lithium foil and the electrolyte was N,N-dimethylacetamide (DMAc) in 1 M LiNO.sub.3. 0.1 M 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as an oxygen evolution reaction mediator was added to the electrolyte.

Example 4: Fabrication of Lithium-Air Battery Using the NiS.SUB.2 .Catalyst

[0054] Lithium-air battery was fabricated in the same manner as in Example 3, except that Super P carbon black was not used and the NiS.sub.2 catalyst (100 wt %) only was used.

Comparative Example 1: Fabrication of Lithium-Air Battery Using the Ni(OH).SUB.2 .Precursor as Catalyst

[0055] A lithium-air battery was fabricated in the same manner as in Example 3, except that the Ni(OH).sub.2 precursor prepared in Example 1 was used as a catalyst instead of the NiS.sub.2 catalyst.

Evaluation Example 1: Optimal Conditions for Ni(OH).SUB.2 .Precursor Synthesis

[0056] Scanning electron microscopy (SEM) images of the Ni(OH).sub.2 precursors synthesized in Example 1 were analyzed to obtain optimal conditions for Ni(OH).sub.2 precursor synthesis. FIGS. 2a to 2c are scanning electron microscopy (SEM) images of the Ni(OH).sub.2 precursors prepared in Example 1. Specifically, FIG. 2a shows SEM images of the Ni(OH).sub.2 precursors prepared by hydrothermal synthesis of the mixed solutions of different pH values at 200 C. for 4 h, FIG. 2b shows SEM images of the Ni(OH).sub.2 precursors prepared by hydrothermal synthesis of the mixed solutions at 200 C. for 4 h and 12 h. FIG. 2c shows SEM images of the Ni(OH).sub.2 precursors prepared by hydrothermal synthesis of the mixed solutions at 140 C., 170 C., and 200 C. for 12 h.

[0057] The Ni(OH).sub.2 precursor in the form of a sheet was more effectively synthesized when 0.08 mol of nickel acetate was sufficiently dissolved in 100 ml of water as a solvent, an ammonium solution was added thereto to prepare a mixed solution of pH 9.1-9.2, the mixed solution was subjected to hydrothermal synthesis at 170 C. for 12 h, and the reaction product was cleaned with water and ethanol and dried.

Evaluation Example 2: Optimal Conditions for NiS.SUB.2 .Catalyst Synthesis

[0058] Scanning electron microscopy (SEM) images of the NiS.sub.2 catalysts synthesized in Example 2 were analyzed to obtain optimal conditions for NiS.sub.2 catalyst synthesis. FIGS. 3a to 3e are scanning electron microscopy (SEM) images of the NiS.sub.2 catalysts produced in Example 2. Specifically, FIG. 3a shows SEM images of the NiS.sub.2 catalysts produced by annealing at 400 C. for 1 h after the molar ratio of the precursor to the sulfur powder was adjusted to 1:10 and 1:20 and the distance between the precursor and the sulfur powder was adjusted to 10 cm. FIG. 3b shows SEM images of the NiS.sub.2 catalysts produced by annealing at 400 C. for 1 h after the molar ratio of the precursor to the sulfur powder was adjusted to 1:20 and the distance between the precursor and the sulfur powder was adjusted to 7 cm and 10 cm. FIG. 3c shows SEM images of the NiS.sub.2 catalysts produced by annealing at 400 C., 450 C., and 500 C. for 1 h after the molar ratio of the precursor to the sulfur powder was adjusted to 1:20 and the distance between the precursor and the sulfur powder was adjusted to 10 cm. FIG. 3d shows SEM images of the NiS.sub.2 catalysts produced by annealing at 500 C. for 1 h after the molar ratio of the precursor to the sulfur powder was adjusted to 1:20 and the distance between the precursor and the sulfur powder was adjusted to 7 cm and 10 cm. FIG. 3e shows SEM images of the NiS.sub.2 catalysts produced by annealing at 500 C. for 1 h after the molar ratio of the precursor to the sulfur powder was adjusted to 1:10 and 1:20 and the distance between the precursor and the sulfur powder was adjusted to 10 cm.

[0059] As revealed from these SEM images, the NiS.sub.2 catalyst in the form of a sheet was effectively synthesized when the molar ratio of the precursor to the sulfur powder was adjusted to 1:20, the distance between the precursor and the sulfur powder was adjusted to 7-10 cm, and the annealing was performed at 400-500 C. for 1 h.

Evaluation Example 3: Analysis of Structures and Bonding States of the Ni(OH).SUB.2 .Precursor and the NiS.SUB.2 .Catalyst

[0060] The structures of the Ni(OH).sub.2 precursor and the NiS.sub.2 catalyst synthesized under the optimal conditions defined in Evaluation Examples 1 and 2, respectively, were analyzed from their X-ray diffraction (XRD) patterns, SEM images, X-ray photoelectron spectroscopy (XPS) spectra, and transmission electron microscopy (TEM) images. FIG. 4 shows X-ray diffraction (XRD) patterns of the Ni(OH).sub.2 precursor prepared in Example 1 and the NiS.sub.2 catalyst produced in Example 2, FIG. 5 shows scanning electron microscopy (SEM) images of the Ni(OH).sub.2 precursor prepared in Example 1 and the NiS.sub.2 catalyst produced in Example 2, FIG. 6 shows X-ray photoelectron spectroscopy (XPS) spectra of the Ni(OH).sub.2 precursor prepared in Example 1 and the NiS.sub.2 catalyst produced in Example 2, and FIG. 7 shows transmission electron microscopy (TEM) images, high-resolution TEM images, and fast Fourier transform (FFT) patterns of the Ni(OH).sub.2 precursor prepared in Example 1 and the NiS.sub.2 catalyst produced in Example 2.

[0061] Referring to the XRD patterns of FIG. 4, the precursor obtained by hydrothermal synthesis showed a pristine Ni(OH).sub.2 phase (the bottom of FIG. 4) and the Ni(OH).sub.2 was entirely converted to NiS.sub.2 after the reaction with sulfur (the top of FIG. 4).

[0062] The SEM images of FIG. 5 reveal that the precursor had a thin, sheet-like two-dimensional structure (the left of FIG. 5) and the NiS.sub.2 catalyst had a sheet-like two-dimensional structure similar to that of the precursor.

[0063] The chemical bonding states of the compounds were confirmed by XPS analysis. As shown in FIG. 6, peaks corresponding to the NiO bond were present in the Ni 2p spectra of the Ni(OH).sub.2 precursor and the NiS.sub.2 catalyst. The presence of the peaks in the NiS.sub.2 catalyst indicates that the surface of the NiS.sub.2 catalyst was slightly oxidized. Peaks corresponding to the NiS bond were observed in the S 2p spectrum of the NiS.sub.2 catalyst but none of the peaks were observed in the S 2p spectrum of the Ni(OH).sub.2 precursor, indicating that the surface of the material was stabilized by elemental substitution.

[0064] The TEM images shown in FIG. 7 were analyzed to confirm crystal plane orientations. The Ni(OH).sub.2 precursor was in the form of nanosheets having a size of 100-200 nm (the top left of FIG. 7), which is in agreement with that observed in the SEM image. As a result of analyzing the high-resolution TEM (HRTEM) image (the top middle of FIG. 7), the Ni(OH).sub.2 precursor was confirmed to have continuous lattice fringes, indicating its high crystallinity. The three sets of lattice fringes had the same interplanar spacing (0.27 nm), which corresponds to the (100) crystal plane of hexagonal Ni(OH).sub.2 structure. The adjacent lattice fringes were at an angle of 120. The FFT pattern on the top right of FIG. 7 reveals that the Ni(OH).sub.2 nanosheets had a single crystalline structure and their {001} crystal plane was exposed.

[0065] The TEM image on the bottom left of FIG. 7 reveals that the NiS.sub.2 catalyst was in the form of a sheet and there was no morphological change between the NiS.sub.2 catalyst and the Ni(OH).sub.2 nanosheets. The HRTEM image on the bottom middle of FIG. 7 shows that the NiS.sub.2 nanosheets had clear lattice fringes and high crystallinity over the entire area. The two sets of lattice fringes had the same interplanar spacing below an angle of 90, which corresponds to the {200} crystal plane of cubic NiS.sub.2. Referring to the FFT pattern on the bottom right of FIG. 7, the NiS.sub.2 nanosheets had a single crystalline structure and their {200} crystal plane was exposed.

Evaluation Example 4: Characterization of the Lithium-Air Batteries

[0066] The lithium-air batteries fabricated in Example 3 and Comparative Example 1 were characterized to evaluate the effects of the NiS.sub.2 catalyst. FIG. 8 shows the results of cyclic voltammetry for oxygen reduction/resolution reactions (ORR/OER) in the lithium-air batteries fabricated in Example 3 and Comparative Example 1, FIG. 9 compares the performance of the lithium-air battery fabricated in Example 3 with that of the lithium-air battery fabricated in Comparative Example 1, FIG. 10 shows scanning electron microscopy (SEM) images of the NiS.sub.2 catalyst after discharging and charging of the lithium-air battery fabricated in Example 3, FIG. 11 shows the cycle stability of the lithium-air battery fabricated in Example 4, and FIG. 12 compares the life characteristics of the NiS.sub.2 catalyst produced in Example 2 with those of conventional transition metal sulfide compositions.

[0067] In FIG. 8, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were confirmed by cyclic voltammetry (CV). As a result, the lithium-air battery fabricated in Example 3 showed higher reaction current densities than the lithium-air battery fabricated in Comparative Example 1.

[0068] FIG. 9 compares the characteristics of the lithium-air batteries fabricated in Example 3 and Comparative Example 1. The lithium-air battery of Example 3 had a full discharge capacity of 22500 mAhg.sup.1, which was found to be higher than that (18900 mAhg.sup.1) of the lithium-air battery of Comparative Example 1. When the characteristics of the lithium-air batteries were evaluated at a scan rate of 500 mAg.sup.1 with a limited capacity, the overpotential of the lithium-air battery of Example 3 was found to be relatively low compared to that of the lithium-air battery of Comparative Example 1.

[0069] FIG. 10 shows SEM images of NiS.sub.2 after discharge and charge. The images confirm the morphology of the peroxide. The peroxide was produced in the form of thin sheets during discharge, and thereafter, it disappeared completely during charge.

[0070] The lithium-air battery fabricated using the 100 wt % NiS.sub.2 catalyst without the addition of a conductive material for the ink preparation in Example 4 was characterized. The results are shown in FIG. 11. The lithium-air battery sustained 235 cycles with a limited capacity of 1000 mAhg.sup.1.

[0071] The cycle life characteristics of conventional transition metal sulfide compounds were compared with those of the NiS.sub.2 compound. Referring to FIG. 12, the cycle life characteristics of the NiS.sub.2 compound were superior to those of the conventional compounds.

[0072] Although the present invention has been described herein with reference to the specific embodiments, these embodiments do not serve to limit the invention and are set forth for illustrative purposes. It will be apparent to those skilled in the art that modifications and improvements can be made without departing from the spirit and scope of the invention.

[0073] Such simple modifications and improvements of the present invention belong to the scope of the present invention, and the specific scope of the present invention will be clearly defined by the appended claims.