Method for preparing electrochromic film through spontaneous cathode reduction driven by potential difference

Abstract

The invention relates to a method for preparing an electrochromic film through spontaneous cathode reduction driven by potential difference. The method comprises the following steps: preparing a precursor deposition solution of the electrochromic film; preparing a reference solution, injecting the reference solution into the single salt bridge, and inserting a metal electrode; immersing the conductive substrate and the single salt bridge into the precursor solution at the same time, and connecting the conductive substrate and the metal electrode to perform spontaneous cathode reduction; and after standing for a period of time, taking out the conductive substrate, and drying to obtain the electrochromic film. Compared with the prior art, the method disclosed by the invention does not need an external power supply, is low in energy consumption, simple in preparation process and low in cost, and has an industrial application prospect of preparing the film on the surface of the conductive substrate.

Description

Technical Field

[0001] This invention belongs to the field of electrochromic thin film preparation technology, and in particular relates to a method for preparing electrochromic thin films by potential difference driven spontaneous cathodic reduction. Background Technology

[0002] Electrochromism refers to the continuous and reversible change in the optical properties of certain materials caused by the simultaneous insertion or extraction of ions and electrons under an applied voltage, which visually manifests as a change in the material's color.

[0003] Tungsten oxide is the earliest discovered inorganic electrochromic material and currently the most widely studied cathode coloring material. Its electrochromic phenomenon was first discovered by American scientist SK Deb in 1969, and based on this, the world's first electrochromic device was fabricated. The most common form of tungsten oxide is tungsten trioxide (WO3), a transition metal oxide that can reversibly change between transparent and blue. It possesses advantages such as high contrast, fast response speed, and good cycle stability, showing promising application prospects in electrochromic smart windows and display devices. Currently, the main methods for preparing tungsten oxide thin films include magnetron sputtering, hydrothermal methods, and electrodeposition. For example, K. Pandurangarao et al. reported a tungsten oxide thin film prepared by magnetron sputtering, with a light modulation amplitude of approximately 65% ​​at 700 nm (Optical Materials, 2020, 101, 109791). CN07188236A discloses a tungsten oxide nanoribbon structure electrochromic thin film and its hydrothermal preparation method, with the film showing a light modulation amplitude of 51% at 800 nm. Jeongsoo Park et al. reported a tungsten oxide thin film prepared by electrodeposition with an optical modulation amplitude of 48.5% at 700 nm (Ceramics International, 2024, 50, 28762).

[0004] Prussian blue (PB) was first discovered accidentally in 1706 by Johann Jacob Diesbach, a German paint worker. Due to its stable blue hue and good lightfastness, PB quickly became an important pigment in painting, dyeing, and other fields, promoting the widespread application of blue pigments. Neff first reported the electrochemical behavior and electrochromic properties of Prussian blue. As a typical anodic coloring material, PB films can undergo reversible changes from colorless and transparent to blue under an electric field, showing broad application potential in display technology, smart windows, and other fields. Currently, the commonly used methods for preparing PB films are mainly hydrothermal and electrodeposition methods. For example, Qian Jianghua et al. first reported a PB film prepared by a hydrothermal method, which showed a light modulation amplitude of 44.9% at 680 nm (Solar Energy Materials and Solar Cells, 2018, 177, 9). V. Bayzi Isfahani et al. reported a PB thin film prepared by electrodeposition with an optical modulation amplitude of 55.36% at 555 nm (Electrochimica Acta, 2019, 304, 282).

[0005] In summary, the preparation of tungsten oxide and Prussian blue thin films still relies on traditional processes such as magnetron sputtering, hydrothermal deposition, and electrodeposition. While magnetron sputtering allows for precise control of film thickness and density, the equipment is complex, requiring high-precision magnetic field control and vacuum systems, resulting in high costs and energy consumption. Hydrothermal deposition typically operates under high temperature and pressure, leading to high energy consumption and hindering large-scale industrialization. Although electrodeposition is usually performed at room temperature and atmospheric pressure, eliminating the need for high-temperature and high-pressure environments, it still requires expensive electrochemical workstations and consumes electrical energy. This clearly contradicts the purpose of electrochromic technology as an effective building energy-saving technology.

[0006] Therefore, there is an urgent need to develop a technology for preparing electrochromic thin films that is simple to synthesize, requires no expensive equipment, and does not consume electrical energy. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies, such as the complex synthesis of electrochromic thin films and the need for external power supply, by providing a method for preparing electrochromic thin films through potential difference-driven spontaneous cathodic reduction.

[0008] The objective of this invention can be achieved through the following technical solutions:

[0009] The first aspect of this invention provides a method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction, the method comprising the following steps:

[0010] Preparation of precursor deposition solution for electrochromic thin films;

[0011] Prepare a reference solution, inject it into a single salt bridge, and insert a metal electrode.

[0012] The conductive substrate and the single salt bridge were simultaneously immersed in the precursor solution, and the conductive substrate and the metal electrode were connected to perform spontaneous cathodic reduction. After standing for a period of time, the conductive substrate was removed and dried to obtain an electrochromic film.

[0013] Furthermore, the electrochromic film is made of any one of transition metal oxides, Prussian blue and polyoxometalates, conductive polymers, viologen compounds, and organometallic complexes.

[0014] Furthermore, the electrochromic film is made of tungsten oxide or Prussian blue. The precursor solution for tungsten oxide is a solution of hexavalent tungsten, while the metal electrode in the salt bridge has a strong reducing ability and easily loses electrons to be oxidized. Electrons are transferred along the wire to the precursor solution near the conductive substrate, causing a reduction reaction that deposits the tungsten oxide film on the conductive substrate. The tungsten oxide film is then oxidized again by hydrogen peroxide and nitric acid in the precursor solution. Similarly, the ferric ions in the Prussian blue precursor solution, after reduction to ferrous ions, combine with ferric ions in the solution to form Prussian blue. Therefore, tungsten oxide and Prussian blue films can be prepared on a conductive substrate using a spontaneous cathodic reduction reaction driven by the potential difference between the metal electrode and the precursor solution. Theoretically, any electrochromic material whose preparation process involves a reduction reaction can be prepared using the method of this invention.

[0015] Furthermore, when the electrochromic film is made of tungsten oxide, the preparation method of the precursor deposition solution is as follows: dissolve the tungsten source, hydrogen peroxide and concentrated nitric acid in water, and stir evenly to obtain the tungsten oxide precursor deposition solution.

[0016] Furthermore, the tungsten source is any one or more of sodium tungstate dihydrate, ammonium metatungstate, ammonium paratungstate, and tungstic acid.

[0017] Furthermore, the molar concentration of the tungsten source in the precursor deposition solution is 0.025–0.25 mol / L, preferably 0.05–0.2 mol / L. Excessive tungsten source concentration results in a thicker film and darker color, affecting the light modulation amplitude; conversely, insufficient tungsten source concentration results in a very thin film and a very light color, also affecting the light modulation amplitude.

[0018] Furthermore, the molar concentration of hydrogen peroxide in the precursor deposition solution is 0.03–0.1 mol / L.

[0019] Furthermore, the molar concentration of nitric acid in the precursor deposition solution is 0.06–0.32 mol / L.

[0020] Furthermore, when the electrochromic film is made of Prussian blue, the preparation method of the precursor deposition solution is as follows: potassium ferricyanide, ferric chloride and potassium chloride are dissolved in water and stirred evenly to obtain a tungsten oxide precursor deposition solution.

[0021] Furthermore, the molar ratio of potassium ferricyanide, ferric chloride, and potassium chloride is 1:1:10.

[0022] Furthermore, the molar concentration of potassium ferricyanide in the precursor deposition solution is 0.005–0.1 mol / L, preferably 0.01–0.05 mol / L. Too high or too low a molar concentration of potassium ferricyanide will affect the optical modulation amplitude.

[0023] Furthermore, the reference solution includes any one of potassium chloride solution, sodium chloride solution, lithium chloride solution, and anhydrous sodium sulfate aqueous solution.

[0024] Furthermore, the concentration of the reference solution is 0.1–5.0 mol / L.

[0025] Furthermore, the metal electrode has reducing ability, specifically any one of zinc, aluminum, nickel, and copper, preferably zinc with strong reducing properties.

[0026] Furthermore, the single salt bridge is preferably a ceramic sand core single salt bridge.

[0027] Furthermore, the conductive substrate includes any one of fluorine-doped tin oxide conductive glass, indium tin oxide conductive glass, flexible conductive substrate, aluminum-doped zinc oxide conductive glass, silver nanowire transparent substrate, and copper nanowire transparent substrate, preferably fluorine-doped tin oxide (FTO) conductive glass or indium tin oxide (ITO) conductive glass.

[0028] Furthermore, the conductive surface of the conductive substrate faces the single salt bridge.

[0029] Furthermore, the settling time is 3 to 720 minutes.

[0030] Furthermore, when the electrochromic film is a tungsten oxide film, the settling time is 3 to 12 hours.

[0031] Furthermore, when the electrochromic film is a Prussian blue film, the settling time is 3 to 10 minutes.

[0032] Furthermore, the standing period is carried out at room temperature and normal pressure.

[0033] A second aspect of the present invention provides an electrochromic thin film, which is prepared by any of the methods described above.

[0034] Furthermore, when the electrochromic film is a tungsten oxide film, its light modulation amplitude at wavelengths of 750 nm and 1200 nm is 70.5% and 90.2%, respectively.

[0035] Furthermore, when the electrochromic film is a Prussian blue film, its light modulation amplitude at a wavelength of 700 nm is as high as 81.1%.

[0036] The third aspect of this invention provides an application of an electrochromic thin film in the fabrication of electrochromic devices, specifically applicable to smart windows, displays, camouflage, energy storage, and other applications.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) This invention utilizes the potential difference between an active metal electrode with a certain reducing ability and the deposition solution to drive spontaneous cathodic reduction, directly growing electrochromic thin films on the surface of a conductive substrate, and successfully preparing tungsten oxide thin films and Prussian blue thin films. Compared with traditional physical methods (such as magnetron sputtering, vacuum evaporation, ion plating, etc.) and chemical methods (electrodeposition, hydrothermal methods, etc.), the method of preparing electrochromic thin films in this invention does not require an external power supply, has low energy consumption, and the preparation process is simple and low-cost, showing promising prospects for industrial application in film preparation on the surface of conductive substrates. The obtained thin films can be applied in the field of electrochromism.

[0039] (2) This invention innovatively places a highly reducing metal within a single salt bridge, then places the precursor solution into a beaker and inserts it into a conductive substrate. When the conductive substrate and the metal electrode are connected, the active metal easily loses electrons and is oxidized. Electrons are transferred along the wire to the precursor solution near the conductive substrate, causing a reduction reaction and thus forming a thin film on the conductive substrate. Theoretically, this method can be used for any electrochromic material prepared through a reduction reaction.

[0040] (3) This invention provides a method for preparing electrochromic thin films directly on a conductive substrate by potential difference-driven spontaneous cathodic reduction. By adjusting parameters such as the concentration of sodium tungstate and hydrogen peroxide in the tungsten oxide deposition solution, the concentration of potassium ferricyanide and ferric chloride in the Prussian blue deposition solution, the type of metal rod, and the reaction time, tungsten oxide and Prussian blue thin films can be obtained on different types of conductive substrates.

[0041] (4) The preparation process of the present invention is simple, the raw materials are inexpensive, and the cost is low; the preparation method is carried out at room temperature and pressure, without the need for high temperature and high pressure environment, expensive experimental equipment, and the preparation process does not require external power supply or other energy; the process equipment is simple, and it is relatively easy to quickly achieve controllable preparation of tungsten oxide and Prussian blue films by adjusting the composition of the deposition liquid; the requirements for the experimental environment are low, no volatile gases are generated, and it is harmless to the human body.

[0042] (5) The tungsten oxide thin film prepared by this invention achieves dual-band modulation in both visible and near-infrared wavelengths, with light modulation amplitudes of 70.5% and 90.2% at wavelengths of 750 nm and 1200 nm, respectively. The light modulation of Prussian blue thin films is mainly concentrated in the long-wavelength region of visible light, with a light modulation amplitude as high as 81.1% at a wavelength of 700 nm. In contrast, the light modulation of tungsten oxide thin films prepared by existing technologies such as hydrothermal methods and electrodeposition is mainly concentrated in the visible light region (CN07188236A; Ceramics International, 2024, 50, 28762). Only oxygen-vacant tungsten oxide, doped tungsten oxide, or tungsten oxide nanocrystals can achieve a large near-infrared light modulation amplitude, but it is still less than 90% (Ceramics International, 2021, 47, 31834; Applied Surface Science, 2023, 611, 155711; Advanced Materials, 2024, 36, 2406939). Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the experimental process of the present invention.

[0044] Figure 2 This is a SEM image of the tungsten oxide thin film prepared in Example 1 of the present invention.

[0045] Figure 3 This is a cross-sectional view of the tungsten oxide thin film prepared in Example 1 of the present invention.

[0046] Figure 4 The image shows the XRD pattern of the tungsten oxide thin film prepared in Example 1 of this invention.

[0047] Figure 5 The cyclic voltammetry curves are for the tungsten oxide thin film prepared in Example 1 of this invention.

[0048] Figure 6 This is a diagram showing the color-changing effect of the tungsten oxide thin film prepared in Example 1 of the present invention.

[0049] Figure 7 The transmittance curve is shown for the tungsten oxide thin film prepared in Example 1 of this invention.

[0050] Figure 8 The response time curve at 750 nm is shown for the tungsten oxide thin film prepared in Example 1 of this invention.

[0051] Figure 9 The response time curve at 1200 nm is shown for the tungsten oxide thin film prepared in Example 1 of this invention.

[0052] Figure 10 This is a SEM image of the Prussian blue film prepared in Example 6 of the present invention.

[0053] Figure 11 This is a cross-sectional view of the Prussian blue thin film prepared in Example 6 of the present invention.

[0054] Figure 12 This is a diagram showing the color-changing effect of the Prussian blue film prepared in Example 6 of the present invention.

[0055] Figure 13 The transmittance curve of the Prussian blue film prepared in Example 6 of the present invention.

[0056] Figure 14 The response time curve at 700 nm is shown for the Prussian blue thin film prepared in Example 6 of this invention. Detailed Implementation

[0057] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0058] Unless otherwise specified, the reagents, methods, instruments, and equipment used in this invention are conventional in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0059] This invention utilizes the potential difference between an active metal electrode with a certain reducing ability and a deposition solution to drive spontaneous cathodic reduction, directly growing an electrochromic thin film on the surface of a conductive substrate.

[0060] The experimental apparatus and preparation process of the present invention are as follows: Figure 1 As shown, after connecting the conductive substrate and the metal electrode with a wire, the potential difference between the active metal and the precursor solution drives the metal electrode to lose electrons and be oxidized. The electrons are transferred to the precursor solution near the conductive substrate through the wire, thereby reducing the precursor solution near the conductive substrate and depositing it on the conductive substrate to obtain an electrochromic film.

[0061] Examples 1-5 below are specific embodiments of preparing electrochromic tungsten oxide thin films using the potential difference driven spontaneous cathode reduction method of the present invention.

[0062] Example 1:

[0063] This embodiment provides a method for preparing tungsten oxide electrochromic thin films by potential difference-driven spontaneous cathodic reduction, the specific steps of which are as follows:

[0064] First, at room temperature, 3.3 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O), 1 mL of 35% hydrogen peroxide (H₂O₂), and 2 mL of 16 mol / L concentrated nitric acid (HNO₃) were dissolved in 200 mL of deionized water. After stirring, a precursor solution for preparing tungsten oxide, i.e., the deposition solution, was obtained. A 1 mol / L KCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow strip of zinc was inserted. Then, the ceramic core single-salt bridge and the cleaned FTO conductive glass were simultaneously immersed in the tungsten oxide deposition solution, with the conductive surface of the FTO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the FTO conductive glass and the zinc strip were connected by wires and allowed to stand for 12 hours. Finally, the FTO glass with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the tungsten oxide film with the properties to be tested.

[0065] Figure 2 This is a SEM image of the tungsten oxide film prepared in this embodiment. As shown in the figure, the surface of the film exhibits a porous structure composed of tungsten oxide nanoparticles. This three-dimensional porous structure not only constructs continuous electron transport channels and shortens the ion diffusion path, but also provides more active sites for electrochemical reactions.

[0066] Figure 3 This is a cross-sectional view of the tungsten oxide thin film prepared in Example 1. Figure 3 It can be seen that the film thickness is approximately 360 nm.

[0067] Figure 4 The X-ray diffraction (XRD) pattern of the tungsten oxide thin film is shown. The sharp peaks in the image indicate good crystallinity of the synthesized film. Apart from the peaks of the fluorine-doped tin oxide (FTO) glass (standard card JCPDF 41-1445), the diffraction peaks at 13.957°, 22.718°, 28.172°, and 36.572° correspond to the (100), (001), (200), and (201) crystal planes of hexagonal tungsten trioxide (WO3), respectively, indicating that the tungsten oxide thin film has been successfully grown on the FTO conductive glass.

[0068] The present invention performs the following performance tests on the prepared tungsten oxide thin film:

[0069] The obtained tungsten oxide film was subjected to 0.5 mol·L⁻¹ -1 The performance was tested in a ZnSO4 electrolyte solution using a two-electrode system, with the prepared thin film serving as the working electrode and the zinc sheet as the counter electrode. Figure 5 The cyclic voltammetry (CV) curves of the tungsten oxide thin film show obvious redox peaks.

[0070] Choose 0.2V as the coloring voltage and 1.5V as the fading voltage, such as... Figure 6 As shown, tungsten oxide films can vary between colorless and transparent and blue, achieving a bright (1.5V vs. Zn / Zn) film. 2+ ), Dark (0.2V vs. Zn / Zn) 2+ There are two states.

[0071] An electrochemical workstation was connected to a UV spectrophotometer, and the transmittance curves of the colored and faded states were measured in the wavelength range of 400–1200 nm. The results are as follows: Figure 7 As shown, the light modulation amplitude at 750 nm wavelength is 70.5%, and the light modulation amplitude at 1200 nm wavelength is 90.2%. The electrochromic response time curves of the thin film at 750 and 1200 nm were further tested, and the results are shown below. Figure 8 and Figure 9 As shown. Taking the time required to reach 90% of the change in light modulation amplitude as the response time, the tungsten oxide film obtained in Example 1 was calculated to have a coloring time of approximately 12.5 s and a fading time of approximately 15.4 s in the visible light region (750 nm), between colorless / transparent and blue. In the near-infrared region (1200 nm), the coloring time was approximately 10.4 s and the fading time was approximately 4.5 s.

[0072] Example 2:

[0073] This embodiment provides a method for preparing electrochromic tungsten oxide thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0074] First, at room temperature, 16.5 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O), 1.82 mL of 35% hydrogen peroxide (H₂O₂), and 4 mL of 16 mol / L concentrated nitric acid (HNO₃) were dissolved in 200 mL of deionized water. After stirring, a precursor solution for preparing tungsten oxide, i.e., the deposition solution, was obtained. A 5 mol / L NaCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow aluminum strip was inserted. Then, the ceramic core single-salt bridge and the cleaned FTO conductive glass were simultaneously immersed in the tungsten oxide deposition solution, with the conductive surface of the FTO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the FTO conductive glass and the aluminum strip were connected by wires and allowed to stand for 12 hours. Finally, the FTO glass with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the tungsten oxide film with the properties to be tested.

[0075] Example 3:

[0076] This embodiment provides a method for preparing electrochromic tungsten oxide thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0077] First, at room temperature, 1.65 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O), 0.6 mL of 35% hydrogen peroxide (H₂O₂), and 0.8 mL of 16 mol / L concentrated nitric acid (HNO₃) were dissolved in 200 mL of deionized water. After stirring, a precursor solution for preparing tungsten oxide, i.e., the deposition solution, was obtained. A 2 mol / L LiCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow nickel strip was inserted. Then, the ceramic core single-salt bridge and the cleaned ITO conductive glass were simultaneously immersed in the tungsten oxide deposition solution, with the conductive surface of the ITO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the ITO conductive glass and the nickel strip were connected by wires and allowed to stand for 6 hours. Finally, the ITO glass with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the tungsten oxide film with the properties to be tested.

[0078] Example 4:

[0079] This embodiment provides a method for preparing electrochromic tungsten oxide thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0080] First, at room temperature, 3.3 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O), 1 mL of 35% hydrogen peroxide (H₂O₂), and 2 mL of 16 mol / L concentrated nitric acid (HNO₃) were dissolved in 200 mL of deionized water. After stirring, a precursor solution for preparing tungsten oxide, i.e., the deposition solution, was obtained. A 1 mol / L Na₂SO₄ aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow copper strip was inserted. Then, the ceramic core single-salt bridge and the cleaned ITO conductive glass were simultaneously immersed in the tungsten oxide deposition solution, with the conductive surface of the ITO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the ITO conductive glass and the copper strip were connected by wires and allowed to stand for 3 hours. Finally, the ITO glass with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the tungsten oxide film with the properties to be tested.

[0081] Example 5:

[0082] This embodiment provides a method for preparing electrochromic tungsten oxide thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0083] First, at room temperature, 3.3 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O), 1 mL of 35% hydrogen peroxide (H₂O₂), and 2 mL of 16 mol / L concentrated nitric acid (HNO₃) were dissolved in 200 mL of deionized water. After stirring, a precursor solution for preparing tungsten oxide, i.e., the deposition solution, was obtained. A 0.1 mol / L KCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow zinc strip was inserted. Then, the ceramic core single-salt bridge and the cleaned flexible conductive substrate PET / ITO were simultaneously immersed in the tungsten oxide deposition solution, with the conductive surface of the PET / ITO facing the ceramic core single-salt bridge to facilitate film growth. Next, the PET / ITO and zinc strip were connected by wires and allowed to stand for 12 hours. Finally, the PET / ITO with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the tungsten oxide film with the properties to be tested.

[0084] The performance of Examples 2-5 was tested in accordance with Example 1. The comparison results of the electrochromic properties of each tungsten oxide film are shown in Table 1.

[0085] Table 1 Comparison of the electrochromic properties of tungsten oxide films prepared in Examples 1-5

[0086]

[0087] As can be seen from the table above, the tungsten oxide film prepared in Example 2 has a large thickness and a very dark color, resulting in a smaller light modulation amplitude. This may be because the concentrations of sodium tungstate dihydrate and reference solution NaCl in the precursor deposition solution are too high.

[0088] The light modulation amplitude of the tungsten oxide film prepared in Example 3 is relatively small, which may be due to the low concentration of sodium tungstate dihydrate and the weak reducing power of nickel metal, resulting in a very thin film with a light color, which in turn leads to a small light modulation amplitude.

[0089] The tungsten oxide film prepared in Example 4 was very thin and had a very light color, resulting in a small light modulation amplitude. This may be because copper metal has a weak reducing ability and a short deposition time.

[0090] The tungsten oxide film prepared in Example 5 has slightly inferior performance compared to that in Example 1. This may be because PET / ITO is a flexible substrate with poor adhesion, resulting in poor film performance.

[0091] Examples 1-5 below are specific embodiments of preparing electrochromic Prussian blue thin films using the potential difference driven spontaneous cathodic reduction method of the present invention.

[0092] Example 6:

[0093] This embodiment provides a method for preparing electrochromic Prussian blue thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0094] First, at room temperature, 0.68 g of potassium ferricyanide (K3Fe(CN)6), 0.26 g of ferric chloride (FeCl3), and 1.5 g of potassium chloride (KCl) were dissolved in 200 mL of deionized water. After stirring, a precursor solution, namely the Prussian blue deposition solution, was obtained. A 1 mol / L KCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow zinc strip was inserted. Then, the ceramic core single-salt bridge and the cleaned FTO conductive glass were simultaneously immersed in the Prussian blue deposition solution, with the conductive surface of the FTO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the FTO conductive glass and the zinc strip were connected by wires and allowed to stand for 5 minutes. Finally, the FTO conductive glass with the Prussian blue film deposited was removed, rinsed with deionized water, and allowed to air dry to obtain the Prussian blue film with the properties to be tested.

[0095] Figure 10 This is a SEM image of the Prussian blue thin film prepared in this embodiment. As can be seen from the image, the surface of the Prussian blue thin film consists of particles of varying sizes and is covered with cracks, providing channels for ion insertion and extraction.

[0096] Figure 11 This is a cross-sectional view of the Prussian blue thin film prepared in this embodiment, with a film thickness of approximately 250 nm.

[0097] The testing method for the electrochromic properties of Prussian blue thin films is the same as that for tungsten oxide thin films. Figure 12 The digital photo is colored and faded; the faded state is colorless and transparent, and the color after coloring is blue. Figure 13 Its transmittance spectrum shows a maximum light modulation amplitude of 81.1% at 700 nm. Figure 14 The response time curves are shown, with a fading time of 20.1s and a coloring time of 9.5s.

[0098] Example 7:

[0099] This embodiment provides a method for preparing electrochromic Prussian blue thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0100] First, at room temperature, 6.8 g of potassium ferricyanide (K3Fe(CN)6), 2.6 g of ferric chloride (FeCl3), and 15 g of potassium chloride (KCl) were dissolved in 200 mL of deionized water. After stirring, a precursor solution, namely the Prussian blue deposition solution, was obtained. A 5 mol / L NaCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow aluminum strip was inserted. Then, the ceramic core single-salt bridge and the cleaned FTO conductive glass were simultaneously immersed in the Prussian blue deposition solution, with the conductive surface of the FTO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the FTO conductive glass and the aluminum strip were connected by wires and allowed to stand for 10 min. Finally, the FTO conductive glass with the deposited Prussian blue film was removed, rinsed with deionized water, and allowed to air dry to obtain the Prussian blue film with the properties to be tested.

[0101] Example 8:

[0102] This embodiment provides a method for preparing electrochromic Prussian blue thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0103] First, at room temperature, 0.34 g of potassium ferricyanide (K3Fe(CN)6), 0.13 g of ferric chloride (FeCl3), and 0.74 g of potassium chloride (KCl) were dissolved in 200 mL of deionized water. After stirring, a precursor solution, namely the Prussian blue deposition solution, was obtained. A 2 mol / L LiCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow nickel strip was inserted. Then, the ceramic core single-salt bridge and the cleaned FTO conductive glass were simultaneously immersed in the Prussian blue deposition solution, with the conductive surface of the FTO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the FTO conductive glass and the nickel strip were connected by wires and allowed to stand for 3 minutes. Finally, the FTO conductive glass with the Prussian blue film deposited was removed, rinsed with deionized water, and allowed to air dry to obtain the Prussian blue film with the properties to be tested.

[0104] Example 9:

[0105] This embodiment provides a method for preparing electrochromic Prussian blue thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0106] First, at room temperature, 0.68 g of potassium ferricyanide (K3Fe(CN)6), 0.26 g of ferric chloride (FeCl3), and 1.5 g of potassium chloride (KCl) were dissolved in 200 mL of deionized water. After stirring, a precursor solution, namely the Prussian blue deposition solution, was obtained. A 1 mol / L Na2SO4 aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow copper strip was inserted. Then, the ceramic core single-salt bridge and the cleaned ITO conductive glass were simultaneously immersed in the Prussian blue deposition solution, with the conductive surface of the ITO conductive glass facing the ceramic core single-salt bridge to facilitate film growth. Next, the ITO conductive glass and the copper strip were connected by wires and allowed to stand for 6 minutes. Finally, the ITO conductive glass with the deposited Prussian blue film was removed, rinsed with deionized water, and allowed to air dry to obtain the Prussian blue film with the properties to be tested.

[0107] Example 10:

[0108] This embodiment provides a method for preparing electrochromic Prussian blue thin films by potential difference-driven spontaneous anodic oxidation, as detailed below:

[0109] First, at room temperature, 3.4 g of potassium ferricyanide (K3Fe(CN)6), 1.3 g of ferric chloride (FeCl3), and 7.5 g of potassium chloride (KCl) were dissolved in 200 mL of deionized water. After stirring, a precursor solution, namely the Prussian blue deposition solution, was obtained. A 0.1 mol / L KCl aqueous solution was prepared and injected into a ceramic core single-salt bridge, into which a narrow zinc strip was inserted. Then, the ceramic core single-salt bridge and the cleaned flexible conductive substrate PET / ITO were simultaneously immersed in the Prussian blue deposition solution, with the conductive surface of the PET / ITO facing the ceramic core single-salt bridge to facilitate film growth. Next, the PET / ITO and the zinc strip were connected by wires and allowed to stand for 5 minutes. Finally, the PET / ITO with the deposited tungsten oxide film was removed, rinsed with deionized water, and allowed to air dry to obtain the Prussian blue film with the properties to be tested.

[0110] The comparison results of the electrochromic properties of the Prussian blue films prepared in Examples 6-10 of this invention are shown in the table.

[0111] Table 2 Comparison of the electrochromic properties of the Prussian blue films prepared in Examples 1-5

[0112]

[0113]

[0114] The Prussian blue film obtained in Example 7 may be due to the high concentration of potassium ferricyanide, resulting in a very thick tungsten oxide film that is difficult to completely fade, thus leading to a smaller light modulation amplitude.

[0115] The Prussian blue film prepared in Example 8 was relatively thin, resulting in a lighter color and a smaller light modulation amplitude. This may be due to the slightly weaker reducing power of nickel metal and the shorter deposition time.

[0116] The Prussian blue film prepared in Example 9 was thin and light in color due to the poor reactivity and weak reducing ability of copper metal, resulting in a smaller light modulation amplitude.

[0117] The Prussian blue film prepared in Example 10 was thinner due to poor adhesion to the flexible PET / ITO substrate, and its performance was worse than that of the FTO substrate.

[0118] In summary, this invention utilizes the potential difference between an active metal electrode with a certain reducing ability and the deposition solution to drive spontaneous cathodic reduction, directly growing electrochromic thin films on the surface of a conductive substrate, successfully preparing tungsten oxide and Prussian blue thin films. Compared with traditional physical methods (such as magnetron sputtering, vacuum evaporation, ion plating, etc.) and chemical methods (electrodeposition, hydrothermal methods, etc.), the method for preparing electrochromic thin films in this invention requires no external power supply, has low energy consumption, and is simple and low-cost, showing promising prospects for industrial application in film formation on conductive substrates. The resulting thin films can be applied in the field of electrochromism.

[0119] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction, characterized in that, The method includes the following steps: Preparation of precursor deposition solution for electrochromic thin films; Prepare a reference solution, inject it into a single salt bridge, and insert a metal electrode. The conductive substrate and the single salt bridge were simultaneously immersed in the precursor solution, and the conductive substrate and the metal electrode were connected to perform spontaneous cathodic reduction. After standing for a period of time, the conductive substrate was removed and dried to obtain an electrochromic film.

2. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 1, characterized in that, The electrochromic film is made of any one of the following: transition metal oxides, Prussian blue and polyoxometalates, conductive polymers, viologen compounds, and organometallic complexes.

3. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 1, characterized in that, The electrochromic film is made of tungsten oxide or Prussian blue.

4. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 3, characterized in that, When the electrochromic film is made of tungsten oxide, the preparation method of the precursor deposition solution is as follows: dissolve tungsten source, hydrogen peroxide and concentrated nitric acid in water, and stir evenly to obtain tungsten oxide precursor deposition solution. The tungsten source is any one or more of sodium tungstate dihydrate, ammonium metatungstate, ammonium paratungstate, and tungstic acid. The molar concentration of the tungsten source in the precursor deposition solution is 0.025–0.25 mol / L; The molar concentration of hydrogen peroxide in the precursor deposition solution is 0.03–0.1 mol / L; The molar concentration of nitric acid in the precursor deposition solution is 0.06–0.32 mol / L.

5. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 3, characterized in that, When the electrochromic film is made of Prussian blue, the preparation method of the precursor deposition solution is as follows: dissolve potassium ferricyanide, ferric chloride and potassium chloride in water and stir evenly to obtain a tungsten oxide precursor deposition solution. The molar ratio of potassium ferricyanide, ferric chloride, and potassium chloride is 1:1:10; The molar concentration of potassium ferricyanide in the precursor deposition solution is 0.005–0.1 mol / L.

6. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 1, characterized in that, The reference solution includes any one of potassium chloride solution, sodium chloride solution, lithium chloride solution, and anhydrous sodium sulfate aqueous solution; The concentration of the reference solution is 0.1–5.0 mol / L.

7. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 1, characterized in that, The metal electrode has reducing ability, specifically any one of zinc, aluminum, nickel, and copper; The conductive substrate includes any one of fluorine-doped tin oxide conductive glass, indium tin oxide conductive glass, flexible conductive substrate, aluminum-doped zinc oxide conductive glass, silver nanowire transparent substrate, and copper nanowire transparent substrate.

8. The method for preparing electrochromic thin films by potential difference-driven spontaneous cathodic reduction according to claim 1, characterized in that, The settling time is 3 to 720 minutes.

9. An electrochromic thin film, characterized in that, The electrochromic film is prepared by the method described in any one of claims 1-8.

10. The application of the electrochromic thin film of claim 9 in the fabrication of electrochromic devices.

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