KR102432004B1 - Star shaped electrode for supercapacitor - Google Patents

Abstract

The present invention relates to a star-shaped electrode for a supercapacitor, and provides an electrode comprising a tri-metal oxide of Cu-Mn-Co. The electrode includes a bundle of nanoneedles that are loosely interconnected while a plurality of nanoneedles are stacked in an orderly manner to form layers, wherein the bundle forms a star-shaped microstructure with a hole in the center.

Description

Star shaped electrode for supercapacitor

The present invention relates to an electrode and a method for manufacturing the same, and more particularly to a star-shaped electrode for a supercapacitor and a method for manufacturing the same.

Because of the rapid depletion of fossil fuels due to the enormous energy demand, there is a need for the development of sustainable high-performance electrochemical energy storage devices (EESDs). At the forefront of EESD, Li-ion batteries (LIBs) have aroused the interest of researchers because of fascinating properties such as ultra-high energy density. However, LIBs also exhibited limitations such as low power density, heat generation and limited cycle life. Therefore, LIB cannot exclusively solve all electrical storage challenges and requires the development of EESDs with properties such as fast kinetics of charge propagation, high power density (P) and long lifetime. Considering this, the fast charging speed required for applications such as electric vehicles or high-power (grid-scale electrical frequency regulation) could be solved by high-power electrochemical pseudocapacitors.

Based on the charge storage mechanism, supercapacitors (SC) have been classified into three general classes: pseudocapacitors, electric double-layer capacitors (EDLCs), and hybrid capacitors. EDLC stores electrical charge in the form of a double-layer by adsorption/desorption of ions at the surface electrolyte interface. Pseudocapacitors can be mainly classified as pseudocapacitances controlled by surface-controlled fast redox reactions and pseudocapacitances governed by ion insertion, and can have a much higher specific-capacitance (S _c ) than EDLCs. Conversely, an asymmetric supercapacitor (AS) is assembled using both EDLC and pseudocapacitive electrodes. Because of the contribution of both mechanisms, AS can exhibit improved electrochemical performance.

Nanostructured transition-metal oxides such as Co ₃ O ₄ , CuO and MnO ₂ have been extensively studied as pseudocapacitive materials in terms of exhibiting high theoretical specific-capacitance and long-term cycle stability. Compared to single metal oxides, binary metal oxides have improved redox properties. Liu et al. synthesized CuCo ₂ O ₄ exhibiting improved Sc (1,131 F/ _g ) and cycling stability (79.7% at 5000 cycles) compared to Co ₃ O ₄ (664 F/g and 68% capacity retention). . Chang et al. synthesized binary Mn-Co oxide (MCO) with improved cycle stability (95% capacity retention at 500 cycles) compared to single Mn oxide (75%). Gomes et al. synthesized Co-Mn oxide and obtained high S _c (832 F/g at 20 mV/s). To further improve the electrochemical properties through multiple redox reactions, trimetallic spinel oxides have also been synthesized recently. For example, Sahoo et al. synthesized Fe-Ni-Co oxide as a method of improving the electrochemical performance of Fe-based metal oxides by introducing high-capacity Co and high-conductivity Ni metals. The prepared oxide exhibited a Sc of 867 F/g at 3 A/ _g and excellent cycling stability of 92.3% even after 10,000 cycles. Li et al. reported that the Mn-Ni-Co ternary oxide prepared by them exhibited a Sc of 638 F/g at 1 A/ _g and a cycling stability (93.6% after 6,000 cycles). As these findings indicate, the coexistence of different transition metal ions in trimetal oxides could enhance the efficient multiple redox reactions to further improve the electrochemical performance.

Mn and Co oxides have recently attracted the interest of researchers because of their high theoretical capacities of ∼1370 F/g and 3,560 F/g, respectively. In addition, recent studies revealed the excellent electrochemical function and excellent cycle stability of Co oxide, the wide operating potential range of Mn oxide, and the high electronic conductivity of Cu oxide.

An object of the present invention is to provide an electrode exhibiting electrochemical properties such as improved capacity and excellent cycling stability, and a method for manufacturing the same.

The present invention provides an electrode including a tri-metal oxide of Cu-Mn-Co in order to achieve the above object.

In the present invention, the tri-metal oxide may be CuMnCoO ₄ .

In the present invention, the trimetal oxide may have a spinel structure formed by substituting a portion of Co ²⁺ or Co ³⁺ with Cu ²⁺ or Mn ³⁺ in the main crystal lattice of the Co oxide.

The electrode according to the present invention includes a bundle of loosely interconnected nanoneedles in which a plurality of nanoneedles are stacked in an orderly manner to form a layer, but the bundle may form a star-shaped microstructure having a hole in the center.

In the present invention, each nanoneedle may be composed of tri-metal oxide nanocrystals, the size of the nanocrystals may be on average 9±3 nm, and the nanocrystals may have a single crystal structure.

In the present invention, ordered tunnels or pores may be formed between the nanoneedles, and nanopores may be formed between the nanocrystals.

The electrode according to the present invention can behave as a pseudocapacitive material, and the charge storage mechanism of the electrode can be dominated by fast surface redox reactions in addition to ion insertion/desorption near the surface.

In the present invention, the specific-capacitance (S _c ) of the tri-metal oxide may be 1400 F/g or more at a three-electrode system and a current density of 1 A/g, and the capacity retention may be 90% or more after 5,000 cycles.

In the present invention, the series resistance (RS ) and the charge transfer resistance ( _{R CT} ) of the tri-metal oxide may be 0.5315 Ω or less and 1.0 Ω or less, respectively.

The electrode according to the invention can be used in the electrode of an asymmetric supercapacitor, the specific capacity and energy density and power density of the supercapacitor being 100 F/g or more, 35 Wh/kg or more and 750 W/g at 1 A/g, respectively. kg or greater, and the capacity retention rate may be greater than or equal to 90% after 10,000 cycles.

In addition, the present invention comprises the steps of performing a hydrothermal process after mixing the Cu precursor, the Mn precursor and the Co precursor; and calcining the precursor film that has undergone a hydrothermal process.

In the present invention, the Cu precursor, the Mn precursor, and the Co precursor may be Mn(NO ₃ ) ₂ , Cu(NO ₃ ) ₂ and Co(NO ₃ ) ₂ , respectively.

In the present invention, the hydrothermal process may be performed at 150±30° C. for 8±3 hours.

In the present invention, the calcination may be performed at 350±30° C. for 2±1 hours at a temperature increase rate of 2±1° C./min.

The novel star-shaped trimetal oxide (CMCO) electrode synthesized using the hydrothermal method according to the present invention can exhibit improved capacity and superior cycling stability compared to monometal (CO) and bimetal (MCO) oxide-based electrodes. have. Distinctive morphological features of ordered layered CMCO nanoneedles can significantly increase electrochemically active sites and facilitate material and charge transfer by forming a star shape with a hole in the center. In addition, three transition metal cations in trimetal oxide can contribute to multiple redox reactions. In addition, an asymmetric supercapacitor using a CMCO electrode can exhibit improved capacity and excellent capacity retention, and high energy density and power density.

1 is an XRD pattern of CO, MCO and CMCO.
2 is an XPS spectrum of CMCO, (a) Co 2p, (b) Mn 2p, (c) Cu 2p, and (d) O 1s.
3 is (a) EDX element mapping, (b) TEM, (c) HRTEM, (d) SAED patterns of CMCO.
4 is a FESEM image of CMCO.
5 is a schematic diagram of fast ion insertion and facile electron transport.
6 shows a comparison of synthesized electrodes in terms of (a) CV and (b) GCD curves. Specifically, (c) CV and (d) GCD curves of CMCO.
7 shows (a) cycling stability, (b) Nyquist plot, and (c) IR drop with GCD current density of all electrodes. (d) shows the equivalent circuit used for fitting the Nyquist plot.
Figure 8 shows (a) CV curves in different potential windows at a scan rate of 50 mV/s of AS (CMCO//AC); (b) GCD curves at different potential windows at 3 A/g; (c) CV curves at different scan rates; and (d) GCD curves at different current densities.
9 shows (a) speed performance, (b) Ragone plot, (c) cycling stability, and (d) CV curves obtained at the 1st and 10,000th cycles of the cycling test of AS.

Hereinafter, the present invention will be described in detail.

The electrode according to the present invention is characterized in that it comprises a tri-metal oxide (CMCO) of Cu-Mn-Co. The electrode may consist almost entirely of CMCO and may contain traces of other materials. Specifically, the tri-metal oxide may be CuMnCoO ₄ . In particular, the trimetal oxide may have a spinel structure formed by replacing a part of Co ²⁺ or Co ³⁺ with Cu ²⁺ or Mn ³⁺ in the main crystal lattice of the Co oxide.

4 and 5, the electrode according to the present invention includes a bundle of loosely interconnected nanoneedles in which a plurality of nanoneedles are stacked in an orderly manner to form a layer, but the bundle is a star-shaped microstructure having a hole in the center can be formed

Specifically, the electrode may include a plurality of star-shaped bundles, and each star-shaped bundle may be interconnected. Each star-shaped bundle is composed of a plurality of nanoneedles, and the plurality of nanoneedles are stacked in an orderly vertical direction to form a layer and can be loosely interconnected, and when viewed as a whole, can form a star shape.

Each nanoneedle constituting the bundle may be composed of tri-metal oxide nanocrystals. In this way, the nanocrystals may be interconnected to form a nanoneedle, and the nanoneedles may be stacked in a vertical direction to form a layered structure, and may be interconnected in five horizontal directions to form a star-shaped bundle. The size of the nanocrystals may be on average 9±3 nm, preferably 9±2 nm or 9±1 nm. In addition, the nanocrystals may have a single crystal structure to improve electrical conductivity and the like.

A relatively large hole may be formed in the center of the star bundle, an ordered tunnel or pores may be formed between the nanoneedles, and nanopores may be formed between the nanocrystals. These ordered tunnels and pores can help the fast flow of ions, thus allowing for easy charge storage.

The electrode according to the present invention can exhibit excellent crystallinity, and can contribute to a large surface area and fast charge and mass transfer kinetics. In addition, the above-described unique morphology of the electrode according to the present invention is characterized by a large number of electrochemically active sites, a tunnel between the central hole and the nanoneedles and rapid ion movement through the nanopores between the nanocrystals, and facilitated electrons through the interconnected nanoneedles. It can help with movement.

The electrode according to the present invention can behave as a pseudocapacitive material, and the charge storage mechanism of the electrode can be dominated by fast surface redox reactions in addition to ion insertion/desorption near the surface.

The specific-capacitance (S _c ) of the trimetal oxide in the present invention is at least 1400 F/g, preferably at least 1500 F/g, at least 1600 F/g or at least 1700 F in a three-electrode system and a current density of 1 A/g. It can be greater than /g. The upper limit may be, for example, 1850 F/g or less, 1800 F/g or less, or 1750 F/g or less. In addition, the capacity retention amount of the trimetal oxide may be 90% or more, preferably 92% or more, 94% or more, or 95% or more after 5,000 cycles. The upper limit may be, for example, 99% or less, 98% or less, 97% or less, or 96% or less.

In the present invention, the series resistance (RS ) of the tri-metal oxide may be _0.5315 Ω or less, preferably 0.5310 Ω or less or 0.5309 Ω or less. The lower limit may be, for example, 0.52 Ω or more, 0.53 Ω or more, or 0.5305 Ω or more. In addition, the charge transfer resistance (R _CT ) of the tri-metal oxide may be 1.0 Ω or less, preferably 0.8 Ω or less, 0.6 Ω or less, or 0.55 Ω or less. The lower limit value may be, for example, 0.52 Ω or more, 0.53 Ω or more, or 0.54 Ω or more.

The electrode according to the present invention can be usefully used as an electrode for a supercapacitor, particularly an asymmetric supercapacitor. The specific-capacitance of the supercapacitor to which the electrode according to the present invention is applied may be 100 F/g or more at 1 A/g, preferably 105 F/g or more, 110 F/g or more, or 111 F/g or more. The upper limit may be, for example, 130 F/g or less, 120 F/g or less, or 115 F/g or less.

The energy density of the supercapacitor to which the electrode according to the present invention is applied may be 35 Wh/kg or more at 1 A/g, preferably 36 Wh/kg or more, 38 Wh/kg or more, or 40 Wh/kg or more. The upper limit value may be, for example, 45 Wh/kg or less, 43 Wh/kg or less, or 41 Wh/kg or less.

The power density of the supercapacitor to which the electrode according to the present invention is applied may be 750 W/kg or more at 1 A/g, preferably 760 W/kg or more, 780 W/kg or more, or 790 W/kg or more. The upper limit value may be, for example, 820 W/kg or less, 810 W/kg or less, or 800 W/kg or less.

The capacity retention rate of the supercapacitor to which the electrode according to the present invention is applied may be 90% or more, preferably 91% or more, 92% or more, or 93% or more after 10,000 cycles. The upper limit may be, for example, 97% or less, 96% or less, 95% or less, or 94% or less.

In addition, the present invention comprises the steps of performing a hydrothermal process after mixing the Cu precursor, the Mn precursor and the Co precursor; and calcining the precursor film that has undergone a hydrothermal process.

The Cu precursor, the Mn precursor, and the Co precursor may be Mn(NO ₃ ) ₂ , Cu(NO ₃ ) ₂ and Co(NO ₃ ) ₂ , respectively. The concentration of each precursor may be 1±0.5 mmol in 40 mL of distilled water. In addition, urea and NH ₄ F may be additionally added to the precursor mixture solution. The concentrations of urea and NH ₄ F may be 4±1 mmol and 8±2 mmol, respectively, in 40 mL of distilled water. The solution can be mixed in air for 30±10 minutes with magnetic stirring.

As a substrate for forming the precursor film, Ni-form (NF) or the like may be used. The substrate may be pre-treated with HCl, ethanol and deionized (DI) water and then vacuum dried.

After the substrate is immersed in the precursor solution, the precursor film may be formed through a hydrothermal process. The temperature of the hydrothermal process may be 150±30°C, preferably 150±20°C, 150±10°C or 150±5°C. The time of the hydrothermal process may be 8±3 hours, preferably 8±2 hours, 8±1 hours or 8±0.5 hours.

After the hydrothermal process is completed, the precursor film may be cooled and washed, and then vacuum-dried at 60±20° C. for 12±3 hours.

Finally, the precursor film can be calcined to form the CMCO three-component oxide electrode. The calcination temperature may be 350±30° C., preferably 350±20° C., 350±10° C. or 350±5° C. The calcination time may be 2±1 hours, preferably 2±0.5 hours. In addition, the calcination may be performed at a temperature increase rate of 2±1° C./min, preferably 2±0.5° C./min.

Commercial activated carbon (AC) or the like may be used as the negative electrode of the supercapacitor, and the CMCO according to the present invention may be used as the positive electrode. Filter paper or the like may be used as the separator, and 3±1 M KOH may be used as the electrolyte. The negative electrode can be prepared, for example, by drop-casting onto NF a slurry of AC, carbon black and polyvinylidene difluoride (PVDF) in a mass ratio of 8±0.5:1±0.5:1.

Hereinafter, the present invention will be described in more detail by way of Examples.

[Example]

In the present invention, in order to combine the maximum benefit from the synergistic effect of different transition metals, and to alleviate their limitations, using hydrothermal synthesis and subsequent calcination, a new star-shaped trimetal compound (Spinel Cu-Mn -Co oxide, CMCO) was designed and prepared. In addition, its electrochemical performance was compared with MCO and Co oxide (CO). According to the observations, multiple redox reactions involving three transition metals have a significant effect on the electrochemical properties of CMCO. CMCO had the highest S _c , the highest electrical conductivity and the best cycle life compared to MCO and CO. In addition, the CMCO//activated carbon (AC) asymmetric HS achieved high energy (40.1 Wh/kg) and power density (799 W/kg), thereby demonstrating the superior performance of CMCO as an electrode material.

1. Experiment

1.1 Manufacture of materials

CO, MCO and CMCO nanostructures were synthesized by a facile hydrothermal method according to the following steps: 1 mmol Mn(NO ₃ ) ₂ , 1 mmol Cu(NO ₃ ) ₂ , 1 mmol Co(NO ₃ ) in 40 mL of distilled water. ) ₂ , 4 mmol urea and 8 mmol NH ₄ F were mixed in air for 30 min with magnetic stirring to obtain a CMCO reaction solution. Ni-foam (NF) (1 cm×4 cm) substrates were pre-treated with 3 M HCl, ethanol and deionized (DI) water and then vacuum-dried overnight to remove contaminants and native oxide layers from the surface. The resulting CMCO solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, and the NF substrate was immersed therein. The hydrothermal process was maintained at 150° C. for 8 hours. At the completion of the reaction time, the autoclave was cooled gradually at 20-25 °C. After carefully removing the NF/CMCO precursor, it was washed several times with DI water to remove residual reactants, and then vacuum-dried at 60° C. for 12 hours. Finally, the precursor film was calcined at 350° C. for 2 hours at a heating rate of 2° C./min, allowing complete phase conversion of the precursor to CMCO ternary oxide.

Further, the MCO reaction solution was prepared by mixing 2 mmol Co(NO ₃ ) ₂ , 1 mmol Mn(NO ₃ ) ₂ , 4 mmol urea and 8 mmol NH ₄ F in 40 mL of DI water in air for 30 min with magnetic stirring. got it Then, a CO reaction solution was obtained by mixing 3 mmol Co(NO ₃ ) ₂ , 4 mmol urea and 8 mmol NH ₄ F in 40 mL of DI water in air for 30 minutes with magnetic stirring. The fabrication of MCO and CMCO electrodes followed the same steps as for CMCO.

1.2. characterization

The crystallographic structure of the prepared electrode was investigated using X-ray diffraction (XRD, PAN analysis and X'Pert-PRO) with a Cu-Kα radiation source. The surface chemical composition was examined by X-ray photoelectron microscopy (XPS, Thermo Scientific) with an Al-Kα monochromated radiation source. The structure and morphology of the films were investigated with a field-emission scanning electron microscope (FE-SEM; Hitachi, S-4800). Detailed structural analysis was performed using a high-resolution transmission electron microscope (HRTEM; CM-200, Philips) at an accelerating voltage of 200 kV.

1.3. electrochemical measurement

The electrochemical performance of the synthesized electrodes was evaluated by measuring their cyclic voltammetry (CV) curves and constant current charge-discharge (GCD) curves at an electrochemical workstation (WonA Tech., WBCS3000). Electrochemical properties were investigated in 3 M KOH electrolyte using a three-electrode system using Ag/AgCl and Pt electrodes immersed in KCl as reference and counter electrodes, respectively. CV was performed at different scan rates (5, 10, 20, 30, 50 and 100 mV/s) within the potential range of 0-0.5 V. GCD was performed with different current densities (1, 2, 3, 4, 5 and 10 A/g) within a stable potential window of 0-0.4 V. The mass loads (mg) of the CO, MCO and CMCO electrodes were 1.32, 1.3 and 1.31 mg, respectively.

S _c was calculated using Equation 1 below.

[Equation 1]

S _c = IΔt / mΔV

where I/m(A/g) represents the current density, Δt(s) is the discharge time, and ΔV(V) is the potential window.

For a two-electrode full-cell configuration, a CMCO//AC AS was assembled using commercial AC as the negative electrode and CMCO as the positive electrode. Additionally, filter paper and 3 M KOH were used as separator and electrolyte, respectively. The negative electrode was prepared by drop-casting onto NF a slurry of AC, carbon black and polyvinylidene difluoride (PVDF) in a mass ratio of 8:1:1. The obtained electrode was then vacuum-dried at 70° C. overnight.

The mass load of AC was determined by balancing the charge at each electrode using Equation 2 below.

[Equation 2]

m ₊ /m _- = (C _- ×ΔV _- ) / (C ₊ ×ΔV ₊ )

where C ₊ and C ₋ are S _c of the CMCO and AC electrodes, respectively; ΔV _- and ΔV ₊ represent the potential windows of the positive and negative electrodes, respectively; m ₊ and m ₋ are the masses of the active materials of the positive and negative electrodes, respectively.

The energy density (E, Wh/kg) and P (W/kg) of the full-cell were calculated from the GCD curve of the two-electrode cell using Equations 3 and 4, respectively.

[Equation 3]

E = (C×ΔV ² ) / 2

[Equation 4]

P = E/Δt

2. Results and Discussion

The crystalline phases of the synthesized CO, MCO and CMCO were analyzed by XRD as shown in FIG. 1 . The strong diffraction peaks at 45°, 52.5° and 79° could be read with the NF substrate. CO showed clear diffraction peaks that can be read in the (220), (311), (511), (440) and (531) planes in Co ₃ O ₄ on spinel (JCPDS# 00-042-1467). Notably, the diffraction peaks of MCO and CMCO were consistent with CO. As confirmed by this result, the crystal structures of MCO and CMCO were the same as Co ₃ O ₄ . Also, as these results show, MCO and CMCO were formed by replacing some of Co ²⁺ or Co ³⁺ with Cu ²⁺ or Mn ³⁺ in the CO main crystal lattice. It was noted that the diffraction peaks of both MCO and CMCO shifted slightly compared to CO. This is due to the difference in the metal-ion radius of Cu, Mn and Co, and it was confirmed that the Co atoms were partially substituted with other atoms (Cu and Mn) in the CO crystal lattice to form the spinel structures of MCO and CMCO.

To further confirm the formation of the ternary spinel structure of CMCO, its chemical composition and oxidation state were investigated by XPS. According to the low-resolution XPS spectrum of CMCO, the detected peaks were of Co, Mn, Cu, O, Ni and C, which represent the elements in CMCO. As the main standard, element C was added for peak position correction, and element Ni was due to the Ni-foam substrate. The Co 2p spectrum shown in Fig. 2a showed two spin-orbital components, Co 2p ^3/2 and Co 2p ^1/2 , at binding energies of ˜781.87 and 797.98 eV, respectively, and thus the presence of Co ²⁺ and Co ³⁺ implied. In the Mn 2p spectrum (FIG. 2b), two peaks corresponding to Mn 2p ^3/2 and Mn 2p ^1/2 were observed around 642.69 and 654.29 eV, respectively, indicating the presence of Mn ³⁺ . In the Cu 2p spectrum (FIG. 2c), there were a shake-up satellite peak and two peaks near 934.4 and 954.2 eV, which corresponded to Cu 2p ^3/2 and Cu 2p ^1/2 of Cu ²⁺ . In the O 1s spectrum (Fig. 2d), the two peaks at 529.7 and 531.1 eV can be assigned to the lattice O ₂ associated with the O ₂ -containing oxides with the metal (Cu, Co and Mn) elements and oxygen of OH ⁻ respectively. there was.

Figure 3a shows energy-dispersive X-ray spectroscopy (EDX) elemental mapping of a CMCO film (Figure 3a). Cu, Mn, Co and O were present in the film, confirming the successful preparation of CMCO. As the EDX elemental mapping of the CO and MCO electrodes also shows, only Co and O were present in CO and Co, Mn and O were present in MCO.

Transmission electron microscopy (TEM) images of CMCO nanostructures are shown in FIGS. 3b and 3c. CMCO showed bundles of nanoneedles. Each nanoneedle was composed of small CMCO nanocrystals, with an average crystallite size of about 9 nm. Notably, the bundles of nanoneedles had an orderly stacked structure, with ordered pores and tunnels between the nanoneedles. The well-ordered tunnels and pores aided in the rapid flow of electrolyte ions, thus allowing for easy charge storage. The selected area electron diffraction (SAED) pattern ( FIG. 3d ) showed diffraction points, indicating the single crystal structure of the CMCO nanoparticles. The single crystal structure revealed the absence of grain boundaries in the CMCO nanoparticles, which reduced the scattering centers for charge transfer and improved the electrical conductivity (or charge mobility) of the three-component spinel structure of CMCO. As shown in Fig. 3c, the HRTEM image of CMCO showed clear lattice fringes with an interplanar d spacing of 0.24 nm, which was in good agreement with the (311) plane of spinel CO (JCPDS 00-042-1467). Based on the above discussion, CMCO had numerous ordered tunnels and pores and exhibited excellent crystallinity, which may contribute to large surface area and fast charge and mass transfer kinetics.

FE-SEM was used to analyze the morphological properties of the synthesized electrodes (CO, MCO and CMCO). CO grew into dense nanowires, whereas MCO grew into nanowires with dense nanosheets. As shown in Fig. 4(a and b), the CMCO was composed of bundles of orderedly layered and loosely interconnected nanoneedles. In particular, the bundles formed a star-shaped microstructure with a hole in the center. This unique shape of the CMCO electrode can help: (1) a large number of electrochemically active sites; (2) rapid ion migration through central pores, tunnels between nanoneedles, and nanopores between nanocrystals; and (3) facilitated electron transport through interconnected nanoneedles. These features are schematically shown in FIG. 5 .

The electrochemical behavior of the synthesized electrode material was studied in a three-electrode system using 3 M KOH electrolyte.

CV and GCD tests were performed to investigate the charge-storage performance of Co, Mn-Co binary and Cu-Mn-Co ternary oxide-based electrodes. 6A shows a comparison of CV profiles of CO, MCO and CMCO electrodes at a scan rate of 20 mV/s. The shape of the CV curves of all electrodes did not show obvious redox peaks and were broad in a wide potential range, indicating that charge storage followed a pseudocapacitive mechanism by surface redox reaction like RuO ₂ and MnO ₂ . The CV profile of the CMCO electrode showed a much larger CV area than that of MCO and CO, demonstrating a significant improvement in electrochemical activity.

To investigate the charge storage mechanism in detail, the CV curves of CMCO (Fig. 6c), CO and MCO were measured at different scan rates. In general, Co, Cu and Mn cations have different electrochemical activities due to different valences in the redox reaction, thus resulting in different contributions to the electrochemical performance of the compounds. By using the dependence of the scan rate of the electrode on the CV current, it is possible to determine whether the electrochemical performance is dominated by the diffusion-limiting Faraday current or by the capacitive current. The current of the capacitive material is proportional to the scan rate (i ∝ ν), and its electrode follows a surface redox mechanism, including EDLC-type and pseudocapacitive materials. Conversely, cell-type electrodes that follow an ion insertion/desorption mechanism to/from bulk exhibit diffusion-limited redox currents proportional to the square root of the scan rate (i ∝ ν ^1/2 ). The contributions of capacitive and diffusion-limiting currents were evaluated and plotted by the equations. The shaded envelope in the CV curve represents a purely capacitive current profile, and the current contribution is summarized in the table. As these results clearly indicate, CO, MCO and CMCO were primarily capacitive rather than diffusion-limiting. Therefore, this charge storage mechanism was mainly governed by the following surface-controlled redox reactions:

[Scheme 1]

MO + OH ^- ↔ MOOH + e ^-

In particular, CMCO showed the highest percentage of capacitive behavior among them. The star-shaped morphology of CMCO consisted of bundles of nanoneedles, each composed of interconnections of nanoscale crystallites. Therefore, its distinct morphology facilitated effective inhibition of surface redox reactions and solid-state diffusion.

To confirm the energy-storage mechanism and discuss the CV curves, the GCD curves of CO, MCO and CMCO were measured at a current density of 1 A/g (Fig. 6b). A limited stable potential window (0–0.4 V) was chosen for the GCD test because of the O ₂ evolution side reaction at high potentials. The discharge curve (in GCD) was not completely linear, but did not show a plateau region. Moreover, the CV curve was approximately square with no apparent redox peaks. This indicates that the electrode behaved as a pseudocapacitive material, and the charge-storage mechanism was mainly dominated by fast surface-redox reactions in addition to ion insertion/desorption near the surface. Additionally, the CMCO electrode exhibited the longest discharge time, demonstrating the highest charge-storage capacity. The S _c values of the CO, MCO and CMCO electrodes were calculated using the GCD curve and Equation 1. The S _c (F/g) values at a current density of 1 A/g were 742, 1305 and 1715 F/g for CO, MCO and CMCO, respectively. The theoretical specific-dose of CMCO was also calculated using possible redox reactions. The theoretical S _c value of CMCO was approximately 3003 F/g, and the experimental S _c value of CMCO (1715 F/g) suggested the superior energy storage capacity of CMCO. The GCD curves of the CMCO ( FIG. 6d ), CO and MCO electrodes at different current densities were also evaluated.

Long cycle life is essential for energy storage cells. 7A shows the GCD cycling stability tested at 5 A/g. CO, MCO and CMCO showed capacity retention of 85.7, 89 and 95%, respectively. The improved cycling stability of CMCO could be attributed to the more advanced capacity contribution by the fast surface-redox reaction, since the surface-redox reaction does not require solid-state diffusion into the bulk lattice, and thus the volume The expansion and phase transition inside the lattice were suppressed.

To compare the charge-transfer kinetics of the samples in detail, their resistance parameters were analyzed with an electrical equivalent circuit (Fig. 7d) to measure the Nyquist plot of the samples listed in Table 1 (Fig. 7b). R _S represents the series resistance, R _CT (charge-transfer resistance) represents the Faraday resistance of the redox reaction, W is the Warburg impedance to the diffusion process, and CPE is the integer phase component. Obviously, the R _CT value (0.54188 Ω) of CMCO was much smaller than that of monometal (CO) and dimetal (MCO) oxides. This is because the unique star-shaped morphology provided a large surface area and facilitated charge transfer. Moreover, three transition metal cations in CMCO initiated beneficial multiple redox reactions. The intersection of the curves on the actual impedance axis represents the R _S value, which is derived from the electrolyte resistance and the internal electrode resistance. CMCO had the lowest R _S value (0.53089 Ω) among all electrodes, indicating good electrical conductivity of CMCO. Figure 7c shows the IR (voltage) drop, calculated from the applied current strength (in the GCD cycle) versus the dwell time between the end of charge and the start of discharge. This further confirmed that CMCO had the lowest R _S . Therefore, the remarkably improved supercapacitive performance of CMCOs is conducive to many electrochemically active sites and has a star-shaped morphology that facilitates material and charge transfer; as well as three transition metal cations contributing to multiple redox reactions; and good electrical conductivity.

Table 1 shows the fitted parameters for the EIS of the synthesized electrodes obtained by Zview software.

Sample R _S R _CT CO 0.53198 1.15 MCO 0.55377 1.057 CMCO 0.53089 0.54188

3. Asymmetric Supercapacitor (AS)

To check the practical application of CMCO, an asymmetric AS was assembled using CMCO and AC as anode and cathode, respectively. To determine the operating voltage window for the SC two-electrode full-cell, a series of CV and GCD curves were studied by varying the operating voltage ( FIGS. 8A and 8B ). The area of the CV curve increased with an increase in the operating potential window, indicating an improved E. However, an operating voltage >1.6 V resulted in the release of O ₂ gas. Therefore, 1.6 V was chosen as the operating voltage window to investigate the electrochemical performance of the AS. Figure 8c shows the CV curves of CMCO//AC at different scan rates in the potential range of 0-1.6 V. The shape of the CV curve was not significantly affected by the increased scan rate, thus demonstrating the high-speed performance and good reversibility of the SC cell.

8 shows the GCD curves of AS at various current densities (A/g). An almost symmetrical charge and discharge triangular curve with respect to time was observed, indicating the capacitive properties of the AS and good electrochemical reversibility. AS obtained a high S _c of 111 F/g at 1 A/g (FIG. 9a). A specific capacity of 59 F/g could be maintained even at a high current density of 10 A/g, demonstrating good rate performance. Capacity is CuO//AC (83 F/g at 1 A/g), CuO@MnO ₂ (49.2 F/g at 0.25 A/g), CuCo ₂ O ₄ /CuO//rGO/Fe ₂ O ₃ (0.25 93 F/g at A/g), NiO//rGO (50 F/g at 1 mA/cm2), NiCo ₂ O ₄ //AC (49.3 F/g at 1 A/g), and MnNiCoO ₄ (1 86.7 F/g at A/g), which could be compared favorably with the previously reported oxide-based HS. Figure 9b shows a Ragone plot of the fabricated asymmetric energy storage cell (CMCO//AC). It exhibited a high E of 40.1 Wh/kg with a P of 799 W/kg at a current density of 1 A/g. Even at an increased current density of 10 A/g, with an E of 20 Wh/kg, P was still close to 8,208 W/kg, which was MnO ₂ //GHCS (22.1 Wh/kg at 100 KW/kg), CuO //AC (31.47 Wh/kg at 892 KW/kg), Co ₃ O ₄ @MnO ₂ /NGO (34.83 Wh/kg at 200 W/kg), NiCo ₂ O ₄ //AC (37.4 at 163 W/kg) Wh/kg), NiCo ₂ O ₄ -rGO//AC (23.3 Wh/kg at 320 W/kg), ZnNiCoO ₄ //AC (35.6 Wh/kg at 150 W/kg), and FeNiCoO ₄ //RGO ( 40 Wh/kg at 750 W/kg) higher than other one-, two- and three-component metal oxide-based ASs. Moreover, the cycling stability of CMCO//AC AS at 10 A/g was tested by continuous charge-discharge cycles. As Figure 9c clearly shows, CMCO//AC AS demonstrated a very stable cycling stability of 93% even after 10,000 cycling tests and maintained a nearly 97% Coulombic efficiency (CE). The CV curves at the 1st and 10,000th cycles (Fig. 9d) showed almost identical CV shapes, confirming the excellent cycling stability of CMCO//AC AS due to the fast surface-redox capacitive nature of CMCO. As these results suggest, CMCO is a promising candidate as an electrode material for practical AS applications.

4. Conclusion

A novel star-shaped trimetal oxide (CMCO) was synthesized using a hydrothermal method. Comparison favorably with monometallic (CO) and bimetallic (MCO) oxide-based electrodes, improved capacity (1715 F/g at current density of 1 A/g) and superior cycling stability (after 5,000 cycles at 10 A/g) 95% capacity retention). As these results indicate, the distinct morphological features of the ordered layered CMCO nanoneedles are by forming a star shape with a hole in the center, which significantly increases the electrochemically active sites and facilitates material and charge transfer. did. In addition, three transition metal cations in the trimetal oxide contributed to multiple redox reactions. CMCO//AC AS showed good capacity retention (93%) even after 10,000 cycles with an Sc of 111 F/ _g at 1 A/g. It also exhibited a high E of 40.1 Wh/kg with a P of 799 W/kg at a current density of 1 A/g, demonstrating practical application. This study paved the way for the study of novel transition metals as spinel trimetal compounds, combining their advantages and exploiting the desired electrochemical properties in energy storage applications.

Claims (14)

It contains a trimetal oxide of Cu-Mn-Co,
An electrode comprising a bundle of loosely interconnected nanoneedles in which a plurality of nanoneedles are stacked in an orderly manner to form a layer, wherein the bundle forms a star-shaped microstructure with a hole in the center. According to claim 1,
3 The metal oxide is CuMnCoO ₄ electrode. The method of claim 1,
The tri-metal oxide is an electrode having a spinel structure formed by replacing a part of Co ²⁺ or Co ³⁺ with Cu ²⁺ or Mn ³⁺ in the main crystal lattice of the Co oxide. delete The method of claim 1,
Each nanoneedle is composed of tri-metal oxide nanocrystals, the average size of the nanocrystals is 9±3 nm, and the nanocrystals have a single crystal structure. The method of claim 1,
An electrode in which ordered tunnels or pores are formed between nanoneedles and nanopores are formed between nanocrystals. According to claim 1,
The electrode behaves as a pseudocapacitive material, and the charge storage mechanism of the electrode is governed by a fast surface redox reaction in addition to ion insertion/desorption near the surface. The method of claim 1,
The specific-capacitance (S _c ) of the tri-metal oxide is greater than or equal to 1400 F/g at a three-electrode system and a current density of 1 A/g, and the capacity retention is greater than or equal to 90% after 5,000 cycles. The method of claim 1,
The series resistance (RS ) and charge transfer resistance ( _{R CT} ) of the tri-metal oxide are 0.5315 Ω or less and 1.0 Ω or less, respectively. According to claim 1,
The electrode is used for the electrode of an asymmetric supercapacitor, and the specific capacity and energy density and power density of the supercapacitor are 100 F/g or more, 35 Wh/kg or more, and 750 W/kg or more, respectively, at 1 A/g, the capacity retention rate The electrode is more than 90% silver after 10,000 cycles. performing a hydrothermal process after mixing the Cu precursor, the Mn precursor, and the Co precursor; and
The method for manufacturing an electrode according to claim 1, comprising calcining the precursor film that has undergone a hydrothermal process. 12. The method of claim 11,
Cu precursor, Mn precursor, and Co precursor are each Mn(NO ₃ ) ₂ , Cu(NO ₃ ) ₂ and Co(NO ₃ ) ₂ An electrode manufacturing method. 12. The method of claim 11,
The hydrothermal process is an electrode manufacturing method performed at 150±30° C. for 8±3 hours. 12. The method of claim 11,
An electrode manufacturing method in which calcination is performed at 350±30° C. for 2±1 hours at a temperature increase rate of 2±1° C./min.

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