KR102485540B1 - Electrode for multifunctional smart window - Google Patents

Abstract

The present invention relates to an electrode for a multifunctional smart window, and provides an electrode having a core/shell-type double-layer configuration of a core containing Co(OH) ₂ and a shell containing Ni(OH) ₂ .

Description

Electrode for multifunctional smart window}

The present invention relates to an electrode and a method for manufacturing the same, and more particularly to an electrode for a multifunctional smart window and a method for manufacturing the same.

The increasing consumption of energy based on fossil fuels is raising concerns about global warming and climate change. Since a significant amount of energy is currently used to maintain indoor temperatures, various efforts have been made to increase energy efficiency, including attempts to control the amount of sunlight entering buildings. Because of their ability to change color and transmittance significantly when a weak voltage is applied, smart windows using electrochromic materials have received great attention, and for similar reasons, electrochromic devices are commonly used in displays, dimming mirrors, and electronic paper. In addition, many research efforts are currently devoted to improving the performance of electrochromic devices in light modulation and energy efficiency, encouraged by additional demands in various fields.

Recently, there have been numerous reports of electrochromic materials with fine-tuning optoelectronic properties and additional functions. For example, conjugated polymers with precisely tailored energy gaps have greatly expanded the color gamut of electrochromic devices, and plasmonic nanomaterials, which are only reactive in a specific wavelength region, provide separation for (visible) light and heat (i.e., infrared). control was made possible. In addition to these approaches, various efforts have been made to store energy in electrochromic materials and create multifunctional systems, since electrochromism itself is a phenomenon that depends on the capture and release of ionic species. Transition metal oxides and hydroxides have shown decent performance, and the use of well-designed nanomaterials with advantageous compositions has been found to yield further improvements. However, when electrochromic devices are targeted for large-scale applications, there is still a great need and demand for highly reliable electrochromic materials and their manufacturing methods.

An object of the present invention is to provide an electrode having ultrafast color transition kinetics for an energy storage-capable electrochromic device and excellent energy storage performance in terms of both capacity and speed performance, and a manufacturing method thereof.

In order to achieve the above object, the present invention provides an electrode having a core/shell-type double-layer configuration of a core containing Co(OH) ₂ and a shell containing Ni(OH) ₂ .

In the present invention, the core may have a nanosheet structure, and the shell may uniformly cover the nanosheet.

In the present invention, Co(OH) ₂ may be polycrystalline, and Ni(OH) ₂ may be amorphous.

The electrode according to the present invention may have multi-step color transition characteristics depending on the applied potential, and the color transition may be reversible.

An electrode according to the present invention can change from yellow through green to brown as the applied potential increases.

In the present invention, the transmittance change according to the applied potential change of the double-layer electrode may be greater than the sum of the transmittance change of the Co(OH) ₂ single-layer and the transmittance change of the Ni(OH) ₂ single-layer.

In the present invention, when the applied potential is changed between -0.2 V and 0.5 V, the change in transmittance of the electrode at a wavelength of 500 nm may be 41% or more.

In the present invention, the coloring time (T _c ) according to the color transition of the double-layer electrode may be smaller than the sum of the coloring time of the Co(OH) ₂ single-layer and the coloring time of the Ni(OH) ₂ single-layer, Also, the decolorization time (T _b ) according to the color transition of the double-layer electrode may be smaller than the sum of the decolorization time of the Co(OH) ₂ single-layer and the decolorization time of the Ni(OH) ₂ single-layer.

The coloring time (T _c ) and decolorization time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds according to the present invention may be 2 s or less and 1.2 s or less, respectively, , the coloring time (T _c ) and decolorization time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 20 seconds may be 3.2 s or less and 2.2 s or less, respectively.

A double-layer electrode formed according to the present invention with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds can have a capacitance of more than 1245 F/g at 2 A/g operation, and from 2 A/g to 20 A/g When increasing, the capacitance can be maintained at 90% or more compared to 2 A/g operation, and after 5000 cycles, the capacitance can be maintained at 70% or more compared to the initial cycle. can have capacity.

A double-layer electrode formed according to the present invention with a core deposition time of 10 seconds and a shell deposition time of 20 seconds can have a capacitance of more than 1462 F/g at 2 A/g operation, and from 2 A/g to 20 A/g When increasing, the capacitance can be maintained at 80% or more compared to 2 A/g operation, and the area capacitance can be more than 20 mF/cm 2 at 0.2 mA/cm 2 operation.

Electrodes according to the present invention can be used in electrochromic devices capable of storing energy, smart windows or capacitors.

In addition, the present invention comprises the steps of electrodepositing a Co precursor on a substrate to form a core containing Co(OH) ₂ ; and forming a shell containing Ni(OH) ₂ by electrodepositing a Ni precursor on the core.

In the present invention, the core electrodeposition time may be 5 to 20 seconds, the shell electrodeposition time may be 5 to 30 seconds, and the electrodeposition current may be 0.5 to 2 mA/cm 2 .

In the present invention, the Co precursor may be an aqueous electrolyte containing Co(NO ₃ ) ₂ , the Ni precursor may be an aqueous electrolyte containing Ni(NO ₃ ) ₂ , and the concentration of each precursor may be 0.05 to 0.2 M. there is.

The electrode according to the present invention has a Co(OH) ₂ /Ni(OH) ₂ core/shell-type double-layer configuration formed by sequential electrodeposition of Co(OH) ₂ and Ni(OH) ₂ , thereby providing energy storage It can have ultra-fast color transfer kinetics for possible electrochromic devices. Also, due to the coexistence of electrochromic materials with different redox potentials, the color of the electrode is switchable among yellow, green and brown, and these changes can be completely reversible. In addition, it can have very fast color transition kinetics and excellent energy storage performance in terms of both capacity and speed performance.

Figure 1 (ac) is a schematic diagram showing the morphological characteristics of (a) Co-10, (b) Ni-10 and (c) Co-10/Ni-10. Fig. 1(d) is a plane of Co-10/Ni-10 and Fig. 1(e) is a cross-sectional SEM image. Fig. 1(f) is a low-magnification and Fig. 1(g) is a high-magnification TEM image, and Fig. 1(h) is a SAED pattern of Co-10/Ni-10. Fig. 1(i) is a STEM image, and Fig. 1(jl) is a corresponding elemental map of Co-10/Ni-10 showing atomic distributions of (j) Co, (k) Ni, and (l) O.
2(a) is a CV graph of single- and double-layer hydroxide films in 1.0 M KOH solution. FIG. 2(b) is a transmittance spectrum of Co-10, Ni-10 and Co-10/Ni-10, and FIG. 2(c) is a schematic and digital photograph showing a color change with respect to an applied potential. Fig. 2(d) is the time-dependent transmittance of Co-10/Ni-10 at 500 nm wavelength under applied potential step-by-step, and Fig. 2(e) shows the one-step potential change for calculation of t _c and t _b is the transmittance obtained by Figure 2(f) is a peak current density versus (scan rate) ^1/2 plot obtained from the measured CV graph at various scan rates and corresponding linear fits. 2(g) is a Nyquist plot of Co-10, Ni-10 and Co-10/Ni-10.
3(a) is a schematic diagram showing the construction and operation of a two-electrode electrochromic cell using Co-10/Ni-10 and mesoporous TiO ₂ electrodes. Fig. 3(b) is a digital photograph of a two-electrode cell after applying a wide range of different voltages, and Fig. 3(c) is the corresponding transmittance spectrum. 3(d, e) are time-dependent transmittance of a two-electrode electrochromic cell at a wavelength of 500 nm under (d) step-by-step and (e) 1-step change of applied voltage, and FIG. 3(f) shows t _c and an enlarged version for the calculation of t _b .
Figure 4(ac) is the charge/discharge profiles of (a) Co-10, (b) Ni-10 and (c) Co-10/Ni-10 in 1.0 M KOH solution under various operating rates. Figure 4(d) is a plot of the mass capacitance of the hydroxide electrode versus current density.
Figure 5 (a) is a plane of Co-10 / Ni-20 and Figure 5 (b) is a cross-sectional SEM image. 5(c) is a CV graph of Co-10, Ni-20 and Co-10/Ni-20 in 1.0 M KOH solution. 5(d) is a transmittance spectrum of Co-10, Ni-20 and Co-10/Ni-20, and FIG. 5(e) is a digital photographic image showing a color change with respect to an applied potential. 5(f) is the time-dependent transmittance of Co-10/Ni-20 at a wavelength of 500 nm under a stepwise change of applied potential. Figure 5(g) is the charge/discharge profile of Co-10, Ni-20 and Co-10/Ni-20 in 1.0 M KOH solution at 2 A/g operation, and Figure 5(h) shows the operating current density is the mass capacitance of the hydroxide film.

Hereinafter, the present invention will be described in detail.

The electrode according to the present invention is characterized by having a core/shell-type double-layer configuration of a core containing Co(OH) ₂ and a shell containing Ni(OH) ₂ .

The core may be composed almost entirely of Co(OH) ₂ , and a small amount of materials such as Co ₃ O ₄ may be included. The shell may consist almost entirely of Ni(OH) ₂ , and trace amounts of other materials may be included.

Referring to FIG. 1 and the like, the core may have a nanosheet structure. Specifically, a net structure may be formed by entangling or combining thread-like films in the transverse and longitudinal directions. Spaces may be formed between the net structures, and may have a sheet or film shape when viewed as a whole. The nanosheet may have a nano size in thickness or the like.

The shell may uniformly cover the nanosheet. Specifically, a plurality of particles may be densely packed on the nanosheet of the core to form a layer. OH ⁻ can migrate along the framework of the nanosheet.

The Co(OH) ₂ of the core may be polycrystalline, and the Ni(OH) ₂ of the shell may be nearly amorphous.

The electrode according to the present invention may have a multi-step color transition (switching) characteristic depending on an applied potential, and the color transition may be completely reversible. Specifically, the electrode according to the present invention may change from yellow through green to brown as the applied potential increases.

A double-layer electrode according to the present invention can exhibit a greater change in transmittance than a single-layer electrode. Specifically, the transmittance change according to the applied potential change of the double-layer electrode of the present invention may be greater than the sum of the transmittance change of the Co(OH) ₂ single-layer and the transmittance change of the Ni(OH) ₂ single-layer.

More specifically, when the applied potential is changed between -0.2 V and 0.5 V, the transmittance change of the electrode at a wavelength of 500 nm may be 41% or more, preferably 43% or more, 47% or more, or 50% or more. The upper limit value may be, for example, 90% or less, 80% or less, 70% or less, or 60% or less. When the shell deposition time is longer, the change in transmittance may become larger.

The double-layer electrode according to the present invention can exhibit very fast color transfer kinetics. Specifically, the coloring time (T _c ) according to the color transition of the double-layer electrode of the present invention is smaller than the sum of the coloring time of the Co(OH) ₂ single-layer and the coloring time of the Ni(OH) ₂ single-layer. Also, the decolorization time (T _b ) according to the color transition of the double-layer electrode may be smaller than the sum of the decolorization time of the Co(OH) ₂ single-layer and the decolorization time of the Ni(OH) ₂ single-layer. .

More specifically, the coloring time (T _c ) and discoloration time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds according to the present invention are 2 s (seconds), respectively. meaning) or less and 1.2 s or less, preferably 1.8 s or less and 1.15 s or less, respectively, or 1.6 s or less and 1.1 s or less, respectively. The lower limits of the coloring time and decolorization time may be, for example, 0.5 s or more, 0.6 s or more, 0.7 s or more, 0.8 s or more, or 0.9 s or more, respectively.

In addition, the coloring time (T _c ) and decolorization time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 20 seconds are 3.2 s or less and 2.2 s or less, respectively, preferably 3 s or less and 2.15 s or less, respectively, or 2.8 s or less and 2.1 s or less, respectively. The lower limits of the coloring time and decolorization time may be, for example, 1.5 s or more, 1.6 s or more, 1.7 s or more, 1.8 s or more, or 1.9 s or more, respectively.

The double-layer electrode according to the present invention can have excellent energy storage performance in terms of both capacity and rate performance. Specifically, a double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds according to the present invention may have a capacitance of 1245 F/g or more, preferably 1248 F/g or more at 2 A/g operation. there is. The upper limit value may be, for example, 1300 F/g or less, 1280 F/g or less, or 1260 F/g or less.

In addition, the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds increases the capacitance from 2 A/g to 20 A/g by more than 90% compared to 2 A/g operation, preferably 92 % or higher, 94% or higher, or 96% or higher. The upper limit value may be, for example, 100% or less, 99% or less, or 98% or less.

In addition, the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds can maintain a capacitance of 70% or more, preferably 72% or more or 74% or more compared to the initial cycle after 5000 cycles. The upper limit may be, for example, 80% or less, 78% or less, or 76% or less.

In addition, a double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds has an areal capacitance of 10 mF/cm 2 or more, preferably 12 mF/cm 2 or more or 14 mF/cm 2 or more at 0.2 mA/cm 2 operation. can have The upper limit value may be, for example, 20 mF/cm 2 or less, 18 mF/cm 2 or less, or 16 mF/cm 2 or less.

A double-layer electrode formed according to the present invention with a core deposition time of 10 seconds and a shell deposition time of 20 seconds can have a capacitance of 1462 F/g or more, preferably 1464 F/g or more, at 2 A/g operation. The upper limit value may be, for example, 1500 F/g or less or 1480 F/g or less.

In addition, the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 20 seconds increases the capacitance from 2 A/g to 20 A/g by 80% or more compared to 2 A/g operation, preferably 81 % can be maintained. The upper limit may be, for example, 88% or less, 86% or less, or 84% or less.

In addition, a double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 20 seconds exhibits 20 mF/cm2 or more, preferably 22 mF/cm2 or more, 24 mF/cm2 or more, or 26 mF/cm2 or more at 0.2 mA/cm2 operation. It may have an areal capacitance of cm2 or more. The upper limit value may be, for example, 34 mF/cm 2 or less, 32 mF/cm 2 or less, or 30 mF/cm 2 or less.

The electrode according to the present invention can be usefully used in an electrochromic device capable of storing energy, a smart window, or a capacitor.

In addition, the present invention comprises the steps of electrodepositing a Co precursor on a substrate to form a core containing Co(OH) ₂ ; and forming a shell containing Ni(OH) ₂ by electrodepositing a Ni precursor on the core.

In this two-step sequential electrodeposition, the electrodeposition order is important. When the order is reversed, that is, when the Ni precursor is electrodeposited first to form a core containing Ni(OH) ₂ , the nanosheet structure is less developed, resulting in multi-color transition characteristics. may not be able to show

The core electrodeposition time may be 5 to 20 seconds, preferably 6 to 15 seconds, and more preferably 8 to 12 seconds.

The shell electrodeposition time may be 5 to 30 seconds, preferably 6 to 25 seconds, and more preferably 8 to 22 seconds.

When the shell electrodeposition time is long, the coloring time and decoloring time may increase and the capacity retention rate may decrease, but the change in transmittance and the capacitance may increase.

The electrodeposition current may be 0.5 to 2 mA/cm 2 , preferably 0.6 to 1.5 mA/cm 2 , more preferably 0.8 to 1.2 mA/cm 2 as a cathode current.

The Co precursor may be an aqueous electrolyte containing Co(NO ₃ ) ₂ , and the Ni precursor may be an aqueous electrolyte containing Ni(NO ₃ ) ₂ . The concentration of each precursor may be 0.05 to 0.2 M, preferably 0.06 to 0.15 M, and more preferably 0.08 to 0.12 M.

As the substrate, a fluorine-doped tin oxide (FTO)/glass substrate or the like can be used. The substrate can be sequentially cleaned with the aid of sonication in acetone, ethanol and deionized water prior to electrodeposition.

Electrodeposition of hydroxide can be carried out using a potentiostat in a three-electrode system using a Pt plate counter electrode and an Ag/AgCl (saturated KCl) reference electrode. After electrodeposition, the hydroxide film may be rinsed with deionized water to remove reactants remaining on the surface and dried.

When manufacturing a two-electrode electrochromic device (cell), one electrode may be an electrode according to the present invention, and the other electrode may be a TiO ₂ film electrode. An electrolyte (such as KOH) may be injected and sealed between the two electrodes.

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

[Example]

The present invention provides a double-layer Co(OH) ₂ /Ni(OH) ₂ hydroxide film with ultrafast color transfer kinetics for an energy storage capable electrochromic device. Nanostructured hydroxide films with a core/shell-type bi-layer configuration were prepared by sequential electrodeposition of Co(OH) ₂ and Ni(OH) ₂ within a very short time (20 to 30 seconds in total). Due to the coexistence of electrochromic materials with different redox potentials, the color of the film was switchable among yellow, green and brown, and these changes were completely reversible. As various investigations into the electrochromic properties of the films clearly indicate, they have very fast color transfer kinetics (among the state of the art) and excellent energy storage performance in terms of both capacity and rate performance. In addition, further investigations and discussions were conducted to understand the origin of high performance and kinetics, and systematic studies were conducted in terms of structural morphology and kinetics of charge and mass transport.

1. Experiment

1.1. Fabrication and characterization of electrochromic films and cells

All chemicals used in this study were purchased from Sigma-Aldrich and were used without further purification unless otherwise noted. The FTO/glass (~14 Ω/cm, Asahi Glass) was washed sequentially with the aid of sonication in acetone, ethanol and deionized water for 30 min each. Electrodeposition of hydroxides on FTO/glass was performed using a potentiostat (WonATech WBCS3000) in a three-electrode system using a Pt plate counter electrode and an Ag/AgCl (saturated KCl) reference electrode. After electrodeposition, the hydroxide film was washed with deionized water to remove the reactants remaining on the surface and dried at 60°C. Doctor-blading of colloidal TiO ₂ nanoparticle paste (Ti-Nanoxide T/SP, Solaronix SA) on FTO/glass, followed by heat treatment at 450°C in air for 30 minutes, for a two-electrode electrochromic cell A mesoporous TiO ₂ film was prepared. A 60 μm thick thermoplastic sealant (Surlyn, DuPont) was used for the sandwich-type assembly of the electrodes for the two-electrode cell, and an electrolyte (1.0 M KOH) was injected into the cell through a pre-drilled hole.

1.2. Evaluation of physicochemical and electrochemical properties

The morphology of the hydroxide film was evaluated using SEM (Hitachi S4800) and TEM (Tecnai G2 F30) equipped with EDS equipment. For TEM analysis, a hydroxide obtained by electrochemical deposition for 60 seconds was used because it is difficult to obtain a reliable TEM sample from a hydroxide deposited for 10 seconds due to its very thin thickness. For Co-10/Ni-10, Co(OH) ₂ was electrodeposited for 60 seconds, and Ni(OH) ₂ was deposited for 10 seconds. XRD measurements were performed using an X'Pert PANalytical equipped with a Cu-Kα radiation source, and XPS analysis was performed using a Thermo Scientific K-Alpha ESCA equipped with an Al-Kα radiation source. XANES spectra were obtained using a synchrotron radiation source at the 8C beamline of the Pohang Accelerator Laboratory. Optical measurements were performed using a Scinco S-3100, and electrochemical analysis was carried out in a three-electrode system including a Pt counter electrode and an Ag/AgCl (saturated KCl) reference electrode in a potentiostat (IVIUM COMPACTSTAT.e and WonATech WBCS3000). ) was performed using.

2. Results and discussion

A double-layer Co(OH) ₂ /Ni(OH) ₂ film was prepared by two-step electrodeposition of fluorine-doped tin oxide (FTO)/hydroxide on a glass substrate in sequential electrodeposition; A cathode current (1 mA/cm 2 ) was applied for 10 seconds to an aqueous electrolyte containing 0.1 M Co(NO ₃ ) ₂ 6H ₂ O and an aqueous electrolyte containing 0.1 M Ni(NO ₃ ) ₂ 6H ₂ O, respectively. did The precise deposition mechanism of hydroxide can be summarized as follows.

[Scheme 1]

NO ₃ ^- + 7H ₂ O + 8e ^- → NH ₄ ⁺ + 10OH ^-

[Scheme 2]

M ²⁺ + 2OH ^- → M(OH) ₂ (M: Co or Ni)

Single-layer Co(OH) ₂ and Ni(OH) ₂ films were additionally prepared by one-step (1 mA/cm 2 for 10 seconds) electrodeposition for comparison. These Co(OH) ₂ , Ni(OH) ₂ and Co(OH) ₂ /Ni(OH) ₂ films are hereinafter respectively Co-10, Ni-10 and Co-10/Ni- 10. According to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Co-10 and Ni-10, sheet-like nanostructures were well developed in Co-10, whereas Ni-10 was on FTO/glass. It was a densely deposited film. As schematically shown in Figures 1a-c, the Co-10/Ni-10 film had a nanostructure similar to that of the Co-10 film, and the thickness of the film was approximately the sum of the Co-10 and Ni-10 film thicknesses (Fig. see SEM and TEM in 1d–g). Selected area electron diffraction (SAED) patterns of Co-10/Ni-10 in Fig. 1h show polycrystalline Co(OH) ₂ (and some Co ₃ O ₄ consistent with previous reports on electrodeposited Co(OH) ₂ ) and It showed the coexistence of almost amorphous Ni(OH) ₂ and compared to the patterns of Co-10 and Ni-10. The presence of Co and Ni in Co-10/Ni-10 was further confirmed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) analysis (Fig. 1i-l). To clarify the composition of Co(OH) ₂ and Ni(OH) ₂ in Co-10/Ni-10, X-ray photoelectron spectroscopy (XPS) analysis was performed. The difference between the Ni 2p signals of Ni-10 and Co-10/Ni-10 was not significant; However, the Co 2p peak of Co-10 significantly decreased after the deposition of Ni(OH) ₂ . Considering that XPS is a surface-sensitive method and many signals come from a depth of several nanometers from the surface, Co-10/Ni-10 has a core/shell-type double-layer structure, while Ni(OH) ₂ is nanometer It was evident that the structure Co(OH) ₂ was uniformly covered. X-ray diffraction (XRD) measurements were also performed on the electrodeposited films, but signals from the hydroxides were barely observable, confirming the extremely thin constituent structures of the electrodeposited Co(OH) ₂ and Ni(OH) ₂ .

The electrochemical properties of Co-10/Ni-10 were first investigated by cyclic voltammetry (CV) analysis. As shown in Figure 2a, Co-10 is about 0 V vs. Ag/AgCl exhibited clear redox peaks (all potential values hereinafter are given for Ag/AgCl (saturated. KCl) unless otherwise stated), indicating that the Co ^2+/3+ transition induced by the reaction is due to

[Scheme 3]

Co(OH) ₂ + OH ^- ↔ CoOOH + H ₂ O + e ^-

Similarly, the Ni ^2+/3+ transition (see Scheme 4 for reaction mechanism) peak was evident at ~0.3 V, and both the Co ^2+/3+ and Ni ^2+/3+ redox peaks were Co-10/ It was present in the case of Ni-10.

[Scheme 4]

Ni(OH) ₂ + OH ^- ↔ NiOOH + H ₂ O + e ^-

Co(OH) ₂ in pre-prepared Co-10 and Co-10/Ni-10 underwent an alpha-to-beta phase transition in the first CV cycle, and the color of the film turned to pale yellow, whereas Ni It is noteworthy that (OH) ₂ did not exhibit this kind of behavior. After initial cycling, all hydroxide films exhibited very stable electrochemical properties in repeated CV measurements. On the other hand, there were some differences in the onset and peak potentials of the Co ^2+/3+ and Ni ^2+/3+ redox reactions in single- and double-layer hydroxide films. X-ray absorption near edge structure (XANES) analysis was performed on the Co K-edge and Ni K-edge to see if there is an electronic interaction between Co(OH) ₂ and Ni(OH) ₂ . From the XANES spectra of the hydroxide films, little difference between the electronic structures of Co(OH) ₂ and Ni(OH) ₂ could be observed in single- and double-layer films. Therefore, it could be assumed that the difference in redox peaks was due to the different electrical and structural properties of the films.

The electrochromic properties of the hydroxide films were investigated by in situ optical measurements. According to the transmittance spectra of Co-10, Ni-10 and Co-10/Ni-10 over a wide range of applied potentials in 1.0 M KOH electrolyte, Co ^2+/3+ and/or Ni ^2+/3+ redox peaks There was a noticeable change in the optical response near the potential, and distinct transmittance spectra of the hydroxide film could be obtained at -0.2 V, 0.2 V and 0.5 V (see Fig. 2b). Since both Co ^2+/3+ and Ni ^2+/3+ redox reactions can occur in Co-10/Ni-10, multi-step color changes were observable; It changed from yellow through green to brown as the applied potential increased (see schematic and digital photograph in Fig. 2c). The color transition was completely reversible, as can be seen from the continuous switching of the optical properties upon repeated changes of the applied potential (see Fig. 2d). Then, the electrochromic performance of the hydroxide films was compared by transmittance in the visible light region, and a wavelength of 500 nm was selected for evaluation of various quantitative characteristics. The transmittance changes of Co-10, Ni-10 and Co-10/Ni-10 at 500 nm wavelength were 8.5, 31.9 and 47.5%, respectively, and the applied potential was changed between -0.2 V and 0.5 V. It was noteworthy to observe that the change in Co-10/Ni-10 was greater than the sum of Co-10 and Ni-10, which was due to the different availability of Ni(OH) ₂ , compared to Co(OH) ₂ . It had a greater importance in electrochromic performance. In Co-10/Ni-10, a nanostructured Co(OH) ₂ film (ie, Co-10) served as a substrate for Ni(OH) ₂ electrodeposition, while bare FTO/glass was used for Ni-10. . For this reason, the surface area of Co-10/Ni-10 is larger than that of Ni-10, indicating that a large amount of Ni(OH) ₂ directly contacts the electrolyte in Co-10/Ni-10 and readily participates in the coloring process. it means. In addition, since Co(OH) _{2 acts as a backbone of Ni(OH) 2 in} Co-10/Ni-10, when Co(OH) ₂ is converted to CoOOH at a high potential, the increase in electrical conductivity of Ni( OH) ₂ can also be increased to increase the electrochromic performance in Co-10/Ni-10 synergistically. Based on the transmittance in the colored state (T _c ) and uncolored state (T _b ) at 500 nm wavelength, the (logarithmic) change in optical density (ΔOD) and the coloring efficiency (CEs) were defined and calculated by the following equations.

[Equation 1]

ΔOD = log (T _b /T _c )

[Equation 2]

CE = ΔOD / Q (Q: charge density)

Since fast switching between different coloration/transparency is advantageous for smart window applications, the transition kinetics of the hydroxide films were then closely investigated. The coloration time (t _c ) and decolorization time (t _b ) of the hydroxide film were determined based on the time taken for 90% of the transmittance change during the transfer process. The t _c / t _b of Co-10, Ni-10 and Co-10/Ni-10 were 0.90 s / 0.36 s, 1.22 s / 0.89 s, and 1.45 s / 1.11 s, respectively (at 500 nm wavelength, Fig. 2e see transmittance data). It is noteworthy that t _c and t _b of Co-10/Ni-10 are smaller than the sum of the single-layer hydroxides, indicating a synergistic effect by combining the nanostructured Co(OH) ₂ with the uniform Ni(OH) ₂ shell. clearly prove that there is On the other hand, when comparing t _c and t _b values with those of previous studies, it could be found that the transition kinetics of the electrodeposited hydroxide film of the present invention belong to the state of the art. It is also noteworthy to observe that the electrochromic performance of the hydroxide film has a good memory effect at small signal and open-circuit conditions of the magnitude reduction of light modulation with high reliability over continuous operation.

In Co-10/Ni-10, Ni(OH) ₂ uniformly covers Co(OH) ₂ (based on XPS analysis results as described above), so the charge is Ni(OH) for proper operation of the electrochromic device. ) ₂ to/from Co(OH) ₂ . As is also evident from the XANES spectra, there was negligible electronic interaction between Co(OH) ₂ and Ni(OH) ₂ in the double-layer hydroxide film. Therefore, the charge transfer kinetics of cobalt and nickel hydroxides in their core/shell-type configurations were investigated to better understand the fast transition kinetics of Co-10/Ni-10. CV measurements were performed at various scan rates, and by plotting the square root of the scan rate against the peak potential (Fig. 2f), the coloration and decolorization (i.e., charge transfer) kinetics of Ni(OH) ₂ are very fast (mass transfer-limiting). ) On the other hand, Co(OH) ₂ was relatively slow from the linear correlation. This indicates that the core/shell-type structure of Co-10/Ni-10 is very efficient for the easy transfer of charge, and Co(OH) ₂ provides a nanostructured backbone for Ni(OH) ₂ for the coloring/bleaching process. This is because it undergoes a rapid electrochemical reaction during Additionally, to compare the charge transfer kinetics of single- and double-layer hydroxide films, electrochemical impedance spectroscopy (EIS) analysis was performed. Figure 2g shows a Nyquist plot of the electrodeposited hydroxide film at open-circuit potential and from the diameter of the first semicircle, the charge transfer resistance (R _ct ) at the hydroxide/electrolyte interface can be compared. Although the EIS data were normalized to the geometric area of the electrode, it was noteworthy that the R _ct of nanostructured Co-10 was larger than that of planar Ni-10. This clearly demonstrates that the charge transfer reaction - change in oxidation state by reacting with OH ^{- -} is significantly faster in Ni-10 compared to Co-10. On the other hand, the R _ct of Co-10/Ni-10 was much smaller than that of the single-layer hydroxide film, which is due to the nanosheet-like morphology originating from the Co(OH) ₂ core and the rapid charge transfer of the Ni(OH) ₂ shell. This could be understood as a result of the kinetic advantage. In addition, the validity of the trend of EIS in open-circuit conditions was cross-confirmed by EIS analysis at various applied potentials (−0.2 V, 0.2 V and 0.5 V). Co-10, Ni-10 and Co-10/Ni-10 showed markedly different behavior in the uncolored (Co(OH) ₂ and Ni(OH) ₂ ) and tinted (CoOOH and NiOOH) states, but in the uncolored (Co(OH) 2 and Ni(OH) 2 ) states, EIS spectrum showed the same trend as in Fig. 2g. For further validation of the advantage of the Co(OH) ₂ /Ni(OH) ₂ configuration, a Ni(OH) ₂ /Co(OH) ₂ film was prepared by reversing the order of electrodeposition, and this film was Ni-10/ It was called Co-10. As can be seen from the SEM images, sheet-like nanostructures are less developed in Ni-10/Co-10 compared to Co-10/Ni-10. It is also noteworthy that the Co ^2+/3+ redox peak is extremely small in the CV graph of Ni-10/Co-10. As a result, as can be seen from the photographs, multi-color transitions were not observable from this opposite-constituent hydroxide film. As can be clearly demonstrated from the results of Ni-10/Co-10, Co-10/Ni-10 is very advantageous for the application of electrochromic devices, especially for multi-color change.

To construct a (two-electrode) electrochromic device (see schematic in Fig. 3a) and demonstrate the feasibility of the hydroxide film in an actual device, a transparent counter electrode was prepared by casting TiO ₂ nanoparticles on an FTO/glass substrate. did; TiO ₂ was chosen because it does not absorb visible light, but at the same time it can act as an ultraviolet (UV) filter. Nanoparticles with a size of about 20 nm were used and did not induce light scattering. On the other hand, since TiO ₂ is also widely known for its electrochromic properties under cation intercalation, the transmittance of TiO ₂ films was measured in both colored and decolored states; The effect of TiO ₂ on the transmittance was not significant in the visible light wavelength region. After the inventive electrochromic hydroxide electrode and TiO ₂ counter electrode were assembled in a sandwich-type configuration using a thermoplastic sealant, a 1.0 M KOH electrolyte was injected into the cell. By changing the applied voltage from 0 to 2.0 V, the color could be switched from yellow through green to brown (Fig. 3b), and this color change was reversible. Figure 3c shows transmittance spectra of the two-electrode cell at different voltages, and a 1/2-fold decrease in transmittance was clearly observable within the visible light wavelength region. This suggests high viability and performance of Co-10/Ni-10 in smart window applications, and it is also noteworthy that UV rays are blocked by the presence of TiO ₂ . Repetitive modulation of the applied voltage to the two-electrode device was then performed, from negligible change in transmittance over continuous operation - under step (Fig. 3d) and one-step (Fig. 3e) modulation in voltage - TiO ₂ counter electrode. The high reliability of the electrochromic device using . In addition, the transition kinetics of the two-electrode electrochromic device were investigated (Fig. 3f), and t _c and t _b were 1.67 s and 1.07 s, respectively, which were consistent with the Co- It was similar to that obtained from 10/Ni-10. As these results clearly show, both the double-layer hydroxide film and the TiO ₂ counter electrode of the present invention are very viable for use in smart windows.

Since both Co(OH) ₂ and Ni(OH) ₂ are pseudo-capacitive materials in alkaline electrolytes, the energy storage properties of electrodeposited hydroxide films were investigated. Figures 4a-c show the charge-discharge curves of Co-10, Ni-10 and Co-10/Ni-10 in 1.0 M KOH electrolyte, respectively, and the calculated capacitances in the discharge phase are summarized in Figure 4d. It was noteworthy that there were significant differences in the capacitance and rate performance of Co-10 and Ni-10. The capacitances of Co-10 and Ni-10 were 483.1 F/g and 1244.3 F/g, respectively, at a relatively slow operating current (2 A/g), the difference being the difference between Co(OH) ₂ and Ni(OH) ₂ This was attributed to differences in crystallinity and intrinsic energy storage properties. On the other hand, the rate performance was superior for Co-10 compared to Ni-10. As the operating current was increased ten times (from 2 A/g to 20 A/g), the capacitance of Ni-10 dropped by about 13.8%, while that of Co-10 decreased by 5.7%. This observation was due to the dissimilar structures of Co-10 and Ni-10; The nanosheet-like morphology of Co-10 provides a larger interfacial area than Ni-10. Also as mentioned above, the mass transfer of OH ⁻ does not limit the overall reaction in Co-10, meaning that the Co(OH) ₂ nanostructures do not appear to impede the transport of charge carriers in the electrolyte. Therefore, OH ⁻ is expected to access Co-10 more easily compared to Ni-10, so the rate performance of Co-10 is superior.

On the other hand, Co-10/Ni-10 showed remarkably high performance in terms of both capacity and rate performance. The capacitance of Co-10/Ni-10 was 1248.6 F/g in low-current (2 A/g) operation, which was slightly greater than that of Ni-10. This was all the more surprising because the important site of Co-10/Ni-10 is Co(OH) ₂ whose capacity is smaller than Ni(OH) ₂ in this study. Even when the operating current was increased from 2 A/g to 20 A/g, the capacitance of Co-10/Ni-10 remained ~97.3%. Capacitance values of single- and double-layer hydroxide films at various charge/discharge rates are summarized. A comparison was made between the results of this study and previous literature results, and the performance of supercapacitor electrodes for energy storage and smart window applications was summarized, respectively. Although the capacity of Co-10/Ni-10 was moderate compared to state-of-the-art, the inventive double-layer hydroxide film exhibited nearly 10 times faster electrochromic kinetics (or similar t _b and t _c compared to comparable energy storage performance). 10 times higher capacitance), which clearly indicates that Co-10/Ni-10 is extremely advantageous for energy-storage smart window applications. The coulombic efficiencies of the hydroxide films were then compared, from which some striking observations could be made; Co-10/Ni-10 had higher Coulombic efficiencies than Co-10 and Ni-10, and for all hydroxide films, the Coulombic efficiencies were high when charging and discharging were performed at large current densities, and this observation was consistent with various compounds It was consistent with the results of previous studies on . Upon further increasing the operating rate to 40 A/g, the Coulombic efficiency further increased to nearly 100%, and even under this high current, ~74% of the initial capacitance was maintained after 5000 cycles of operation. Also, since electrochromic films are mainly used in smart windows, and the standard is often determined by area rather than weight, we additionally obtained the capacitance normalized to the geometric area of the hydroxide film. From the charge/discharge curves and rate-dependent capacitance, the advantages of the core/shell-type Co-10/Ni-10 film are even clearer; The areal capacitance of Co-10/Ni-10 was 14.66 mF/cm2 (at 0.2 mA/cm2 operation), whereas the values of Co-10 and Ni-10 were 2.49 and 8.37 mF/cm2 respectively, single- and The capacitance difference of the double-layer hydroxide film became large at high operating current.

To further improve the light modulation and energy storage performance of the hydroxide film, the deposition time of Ni(OH) ₂ was doubled. Single- and double-layer films prepared by 20 s of Ni(OH) ₂ deposition were termed Ni-20 and Co-10/Ni-20, respectively, in a similar manner to the previous films. As shown in the SEM images of FIGS. 5A and 5B, Co-10/Ni-20 had a nanosheet-like morphology similar to Co-10/Ni-10, and Ni(OH) ₂ in Ni-20 was Ni-10 As in the case of , it was a dense layer. The electrochemistry and corresponding electrochromic reaction were then investigated by CV analysis and in situ optical measurements. From the CV graph of FIG. 5c, Co ^2+/3+ and Ni ^2+/3+ redox peaks were observed at potentials similar to Co-10/Ni-10 (i.e., about 0 and 0.3 V), and Ni The current from ^2+/3+ was larger. The lack of electronic interaction between Co(OH) ₂ and Ni(OH) ₂ in Co-10/Ni-20 was also confirmed by XANES analysis as in the case of Co-10/Ni-10. The change in the optical response was most remarkable in the observed redox peak, and accordingly, -0.2 V, 0.2 V, and 0.5 V (same as the case of Co-10/Ni-10) were selected as operating potentials for Co-10/Ni Electrochromic properties of -20 were analyzed. Figure 5d shows the transmittance of Co-10, Ni-20 and Co-10/Ni-20 at various applied potentials. It is noteworthy that within the center of the visible light region (i.e., about 500–600 nm wavelength region), the transmittance change was greater than 50%, which was attributed to the presence of a thick Ni(OH) ₂ layer formed by 20 s electrodeposition. did The color of the double-layer hydroxide film at the fully colored stage was dark brown (see digital photograph shown in Fig. 5e), and the color transition from yellow through green to dark brown was completely reversible, as can be seen from the light modulation in Fig. 5f. As you can see, it was very stable in long-term operation. Since the change in thickness and transmittance of the electrochromic hydroxide layer increased as the deposition time of Ni(OH) ₂ increased, t _c and t _b increased to some extent; The t _c / t _b of Ni-20 and Co-10/Ni-20 were 2.33 s / 1.88 s and 2.66 s / 2.00 s, respectively. Nonetheless, these values are still among the shortest transition times, and together with other experimental observations on Co-10/Ni-20, the electrodeposition method of the present invention provides controlled optical performance within a fairly fast window of transition kinetics. It could be understood as a very effective method for producing an electrochromic film having

Then, the energy storage performance of Co-10/Ni-20 was investigated. Figure 5g shows the charge/discharge curves of Co-10, Ni-20 and Co-10/Ni-20, and the capacitances of these hydroxide films at various operating current densities are summarized in Figure 5h. As the Ni(OH) ₂ layer becomes thicker, the total capacitance of the film goes from 1244.3 F/g for Ni-10 to 1461.8 F/g for Ni-20 under 2 A/g operation and for Co-10/Ni- It increased significantly from 1248.6 F/g of 10 to 1464.0 F/g of Co-10/Ni-20. This change was even more dramatic when the capacitance was normalized to the geometric area of the film, with a 77.3% increase observable for Co-10/Ni-20 when compared to Co-10/Ni-10. On the other hand, Ni-20 and Co-10/Ni-20 each had a lower capacitance retention than Ni-10 and Co-10/Ni-10 at high current operation, a result of which was the thickness of the Ni(OH) ₂ layer. attributed to the increase. However, it was still noteworthy that there was a clear improvement in rate performance when a nanosheet-like structure derived from Co(OH) ₂ was introduced; The capacitance retention rate of Ni-20 was 81.7% when the operating current density increased from 2 A/g to 20 A/g, whereas Co-10/Ni-20 was 93.3%. The areal capacitance of Co-10/Ni-20 was additionally measured. The areal capacitances of Co-10 and Ni-20 were 2.49 and 15.60 mF/cm, respectively, at an operating current of 0.2 mA/cm, but the capacitance increased to 27.66 mA/cm in Co-10/Ni-20, which is indicates that there is a significant synergistic improvement in These observations also clearly demonstrate the advantages of the core/shell configuration in terms of the ability to control the amount of storable energy and the kinetics of charge and discharge.

3. Conclusion

In this study, a double-layer hydroxide film was prepared by sequential electrodeposition of Co(OH) ₂ and Ni(OH) ₂ in a very short time (10 s each) and applied as an electrode material for a multifunctional smart window. As shown, these films allowed very fast switching (among the state of the art) between multiple colors and a wide range of transmittances. In addition, the electrodeposited hydroxide film exhibited good energy storage performance as a capacitor in terms of both capacitance and rate performance. It should be noted that these results are all the more surprising given the simplicity of the procedure for fabricating the electrodes, and there is still great room for further improvement based on the inventive approach by structural or compositional optimization. In addition, the electrode design concept of the present invention, which can be summarized as combining various electrochromic materials with different redox potentials and physicochemical properties, provides important insight into efforts to provide additional functionality to smart windows, which are rapidly growing in the market. believe to do

Claims (15)

It has a core/shell-type double-layer configuration of a core containing Co(OH) ₂ and a shell containing Ni(OH) ₂ ,
An electrode in which the change in transmittance with a change in the applied potential of the double-layer electrode is greater than the sum of the change in transmittance of the Co(OH) ₂ single-layer and the change in transmittance of the Ni(OH) ₂ single-layer. According to claim 1,
An electrode in which the core has a nanosheet structure and the shell uniformly covers the nanosheet. According to claim 1,
An electrode wherein Co(OH) ₂ is polycrystalline and Ni(OH) ₂ is amorphous. According to claim 1,
The electrode has multi-step color transition characteristics depending on the applied potential, and the color transition is reversible. According to claim 4,
An electrode that changes color from yellow through green to brown as the applied potential increases. delete According to claim 1,
An electrode in which the change in transmittance of the electrode at a wavelength of 500 nm is 41% or more when the applied potential is changed between -0.2 V and 0.5 V. According to claim 1,
The coloring time (T _c ) according to the color transition of the double-layer electrode is smaller than the sum of the coloring time of the Co(OH) ₂ single-layer and the coloring time of the Ni(OH) ₂ single-layer,
The decolorization time (T _b ) according to the color transition of the double-layer electrode is smaller than the sum of the decolorization time of the Co(OH) ₂ single-layer and the decolorization time of the Ni(OH) ₂ single-layer. According to claim 8,
The coloring time (T _c ) and discoloration time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 10 seconds are 2 s or less and 1.2 s or less, respectively,
The coloring time (T _c ) and decolorization time (T _b ) according to the color transition of the double-layer electrode formed with a core electrodeposition time of 10 seconds and a shell electrodeposition time of 20 seconds are 3.2 s or less and 2.2 s or less, respectively. According to claim 1,
A double-layer electrode formed with a core deposition time of 10 seconds and a shell deposition time of 10 seconds has a capacitance of greater than 1245 F/g at 2 A/g operation, and a capacitance at an increase of 2 A/g to 20 A/g. An electrode having an area capacitance of 10 mF/cm 2 or more at 0.2 mA/cm 2 operation, maintaining a capacitance of 70% or more of the initial cycle after 5000 cycles, and maintaining 90% or more of 2 A/g operation. According to claim 1,
A double-layer electrode formed with a core deposition time of 10 seconds and a shell deposition time of 20 seconds has a capacitance of greater than 1462 F/g at 2 A/g operation, and a capacitance at an increase of 2 A/g to 20 A/g. An electrode having an areal capacitance greater than or equal to 20 mF/cm 2 at 0.2 mA/cm 2 operation with a retention greater than 80% at 2 A/g operation. According to claim 1,
The electrode is an electrode used in an electrochromic device capable of storing energy, a smart window, or a capacitor. Forming a core containing Co(OH) ₂ by electrodepositing a Co precursor on a substrate; and
An electrode manufacturing method according to claim 1 comprising forming a shell containing Ni(OH) ₂ by electrodepositing a Ni precursor on a core. According to claim 13,
The electrode manufacturing method wherein the core electrodeposition time is 5 to 20 seconds, the shell electrodeposition time is 5 to 30 seconds, and the electrodeposition current is 0.5 to 2 mA/cm 2 . According to claim 13,
The Co precursor is an aqueous electrolyte containing Co(NO ₃ ) ₂ , the Ni precursor is an aqueous electrolyte containing Ni(NO ₃ ) ₂ , and the concentration of each precursor is 0.05 to 0.2 M electrode manufacturing method.

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