Advanced Electrochromic Functionality via Layered Cobalt Oxide Deposition on Tungsten Oxide Electrodes

Abstract

The integration of various transition metal oxides into tungsten oxide (WO₃) has been widely investigated to enhance its electrochromic (EC) performance. This approach aims to address the inherent limitations of individual metal oxides, such as poor durability, inadequate color neutrality, and restricted coloring efficiency and optical properties. The use of mixed metal oxides has emerged as a promising strategy, enabling a synergistic effect that optimizes EC performance and expands the material’s functional capabilities. In this study, we compare single-layer WO₃ films with bilayer WO₃/cobalt oxide (CoO) (denoted as W@C) composite films, focusing on their structural, morphological, and electrochromic properties. Both films were fabricated using the electrodeposition technique, with a consistent number of deposition cycles. Field emission scanning electron microscopy (FESEM) analysis revealed that the WO₃ film presented a tightly packed arrangement of nanogranules. In contrast, the bilayer W@C composite thin film exhibited a highly interconnected and porous granular structure, with morphology evolving into larger spherical aggregates. The optimized bilayer W@C composite demonstrated exceptional electrochromic performance, achieving an optical modulation of 85.0% at 600 nm and a significantly improved coloration efficiency of 96.07 cm²/C. Stability tests confirmed its remarkable durability, showing only a 1.05% decrease in optical contrast after 5000 s of operation. Additionally, a prototype electrochromic device based on the W@C film demonstrated an optical modulation of 52.13% and outstanding long-term stability, with minimal degradation in performance.

1. Introduction

The swift growth of the global economy has been accompanied by a significant increase in energy demand and environmental degradation, creating substantial barriers to achieving sustainable development. In light of these challenges, considerable research has been dedicated to developing strategies that optimize resource use while reducing wasteful consumption [1,2,3]. A particularly promising result of these efforts is the development of electrochromic devices (ECDs), which have shown transformative potential in various applications, such as energy-efficient building technologies, bi-stable displays, tunable optical filters, and adaptive color-switching materials. These innovations mark a crucial step forward in achieving sustainable and energy-efficient solutions in an increasingly fast-paced technological landscape [4,5,6,7]. Electrochromism refers to the ability of materials to reversibly alter their optical properties under the influence of an external voltage. This process involves a redox-driven chemical transition that modifies the electron states, particularly the molecular π- or d-electrons. Such electrochemical behavior allows for precise control over the material’s transmittance, reflectivity, and color in response to incident light. The reversible color changes not only emphasize the dynamic nature of the material but also enable the development of ECDs that can actively regulate environmental factors, such as light transmission and thermal management. By adjusting the material’s optical properties, ECDs can significantly reduce energy consumption, potentially saving 40%–50% in energy use by dynamically controlling the amount of light and heat entering buildings and vehicles [8]. EC materials encompass a wide variety of substances, typically classified into categories such as transition metal compounds, conductive polymers, and non-oxide inorganic materials. Among these, transition metal-based compounds have proven to be highly promising candidates for EC applications due to their exceptional electrochemical properties. Notable examples include tungsten oxide (WO₃), nickel oxide (NiO), niobium oxide (Nb₂O₅), titanium oxide (TiO₂), manganese oxide (MnO₂), cobalt oxide (CoO), and vanadium oxide (V₂O₅) [9,10,11,12,13]. These metal-based materials are distinguished by their unique electronic, optical, and redox properties, allowing for precise control over their optical responses, such as color and transparency, when exposed to an external electrical stimulus. The ability of these compounds to undergo reversible changes in their electronic structure and light absorption properties makes them ideal candidates for integration into devices that require dynamic modulation of optical performance [14,15,16]. WO₃ is a leading material for EC applications, renowned for its exceptional electrochemical performance, including excellent optical modulation and high coloration efficiency. It can undergo reversible electrochemical reactions, enabling it to switch between colors, such as from transparent to blue, when a voltage is applied. This ability to modulate its optical properties makes WO₃ an ideal candidate for ECDs, including smart windows, mirrors, and energy-efficient displays [17]. One of the key advantages of WO₃ is its high stability and durability under cycling conditions, which contribute to its long-term performance. It also offers relatively good optical contrast and a broad electrochemical window, making it versatile for a range of EC applications. However, WO₃ faces challenges, such as relatively slow switching speeds due to its low conductivity, which can limit the electron transfer rate. While WO₃ provides good optical modulation, its optical contrast during switching may not always meet the requirements of applications that demand significant color changes [18]. To overcome these limitations, researchers have explored combining WO₃ with other transition metal oxides (TMOs) to enhance its EC properties. The integration of WO₃ with metal oxides such as TiO₂, V₂O₅, and CoO can improve color neutrality, cycling stability, and coloring efficiency. These combinations generate synergistic effects, enhancing ionic conductivity, reducing the activation energy for ion transport, and optimizing charge storage. These improvements result in faster switching speeds, better optical contrast, and increased coloring efficiency. For example, the bilayer deposition of CoO can enhance the stability, durability, and electrochemical performance of WO₃, leading to improved cycle life and more efficient performance in ECDs [19,20,21,22].

Gill et al. fabricated a TiO₂ nanotube and WO₃ composite for EC applications, using a TiO₂ paste as an adhesion layer between the TiO₂ and FTO glass. The composite showed enhanced EC performance with higher ion storage capacity, better stability, improved EC contrast, and longer memory time compared to pure WO₃ and TiO₂ [3]. Poongodi et al. developed vertically oriented WO₃ nanoflake arrays via a template-free electrodeposition method, demonstrating superior EC performance. The material exhibited a high optical modulation of 68.89% at 550 nm, rapid response times (t_b = 1.93 s, t_c = 2.87 s), a high coloration efficiency of 154.93 cm²/C, and excellent cyclic stability over 2000 cycles without degradation, highlighting its potential for EC applications [23]. Venugopal et al. created EC bilayers by overcoating MnO₂ on WO₃ films, improving cyclic stability and charge storage without affecting EC functions. The devices exhibited outstanding visible and IR blocking (~98% beyond 600 nm) [24]. Lee et al. developed MnO₂/Ni(OH)₂ electrodes with fast switching times (2.66 s for bleaching, 2.72 s for coloring) and high transmittance retention. The hybrid structure improved EC performance, achieving rapid color change and excellent performance in a two-electrode system [25]. Zhao et al. developed TiO₂ nanorod arrays strengthened WO₃ nano-trees (TWNTs), showing excellent EC performance with 77.35% transmittance retention after 10,000 cycles. The TWNTs film achieved 79.5% ΔT at 633 nm, fast switching (1.9 s/14.8 s), and high coloration efficiency (443.4 cm²/C), demonstrating superior EC stability and performance [26]. Mishra et al. developed TiO₂–Co₃O₄ core–shell nanorod arrays with stable color change from transparent (sky blue) to opaque (dark brown), achieving a coloration efficiency of 91 cm²/C. The structures showed excellent cyclic stability in EC applications, making them ideal for future electronic devices [27]. The literature on W@C bilayer films is limited. Therefore, the EC properties of W@C require further study because most of the work has been on bare WO₃ with much less focus on the combination with CoO.

This manuscript presents an innovative approach for fabricating W@C bilayer composite thin films, utilizing a novel combination of simple electrodeposition techniques. This study provides a thorough analysis of the W@C bilayer composite thin films, examining the effects of these films on their structural, morphological, and EC properties. The findings offer valuable insights into the films’ characteristics and their potential applications.

2. Experimental Section

2.1. Reagents and Materials

All the materials used for the synthesis were of analytical grade and required no additional purification. Fluorine-doped tin oxide (FTO) glass substrates were obtained from MTI Co., Ltd., Seoul, Republic of Korea. Before deposition, the FTO glass was thoroughly cleaned using ultrasonication in ethanol, acetone, and deionized (DI) water for 15 min each. Sodium tungstate (St. Louis, MO, USA) dihydrates (Na₂WO₄·2H₂O), 30% hydrogen peroxide (St. Louis, MO, USA) (H₂O₂), nitric acid (St. Louis, MO, USA) (HNO₃), cobalt chloride (CoCl₂), sodium sulfate (Na₂SO₄), sodium hydroxide (NaOH), lithium perchlorate (LiClO₄), and propylene carbonate (PC), etc.

2.2. Synthesis of WO₃ and W@C Bilayer Composite Thin Films

WO₃ thin films were synthesized using a modified sol-based electrodeposition technique. First, a 150 mL aqueous solution of Na₂WO₄·2H₂O (25 mM) was prepared using double-distilled water (DDW) and stirred for 15 min to ensure uniform mixing. Subsequently, 1 mL of 30% H₂O₂ was added, resulting in a deep yellow coloration, confirming the formation of peroxytungstic acid (PTA). The solution was further modified by the gradual addition of HNO₃ while maintaining the temperature at 45 °C. Once the reaction was complete, the PTA solution was cooled to room temperature. For electrodeposition, the prepared PTA solution was used as the electrolyte. The process was carried out using a three-electrode system, where the FTO substrate served as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. A potential of ±1 V was applied for 20 cycles to deposit WO₃ onto the FTO substrate. After deposition, the films were rinsed thoroughly, dried, and annealed at 450 °C.

To fabricate W@C bilayer composite thin films, CoO was electrodeposited onto the pre-synthesized WO₃ surface. The WO₃/FTO substrate, Pt wire, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. The electrodeposition process was conducted at room temperature under potentiostatic conditions. The WO₃/FTO electrode was immersed in a 60 mL electrolyte containing 25 mM CoCl₂, followed by the addition of Na₂SO₄ and NaOH, which resulted in a reddish-colored solution. The deposition was performed using a CV approach within a potential range of −1.2 V to 1 V, with a sweep rate of 50 mV/s for 20 electrodeposition cycles. After deposition, the W@C bilayer composite thin films were washed thoroughly with DI water and absolute ethanol to remove residual impurities. The films were then dried at 60 °C and annealed at 450 °C to improve their structural and electrochemical properties. The final bilayer composite thin film material was labeled as W@C and was consistently referred to as such throughout this manuscript. Figure 1 presents a schematic diagram of the potential process for forming W@C bilayer composite thin films. This synthesis approach demonstrates the fabrication of both single-layer WO₃ thin films and W@C bilayer composite thin films, providing a foundation for their EC applications.

3. Electrochromic Device Fabrication

This study explored the practical application of the W@C bilayer composite thin film in ECDs. The device was assembled with a layered structure of Glass/FTO/W@C/LiClO₄ + PC/FTO/Glass, with dimensions of 3 × 4 cm². In this configuration, the FTO glass substrate functioned as the counter electrode, while the W@C thin film deposited on FTO acted as the EC active layer. A 1 M LiClO₄ + PC electrolyte was introduced between the active-layer-coated FTO and the bare FTO/glass substrate, ensuring efficient ionic transport. To maintain structural integrity and prevent electrolyte leakage, the entire assembly was securely sealed using transparent double-sided adhesive tape (Scotch Brand Tape, 3M, Haverhill, MA, USA).

4. Material Characterization

The crystal structure of the samples was analyzed using X-ray diffraction (XRD) with Cu-Kα radiation (PAN Analytical, Almelo, The Netherlands). The surface morphology and elemental composition of the electrodes were examined by field-emission scanning electron microscopy (FE-SEM, S4800 HITACHI, Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS). Prior to FE-SEM and EDS measurements, the samples were coated with a thin layer of platinum via sputtering. The chemical composition and valence state of the materials were determined using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, Cheshire, UK). Electrochemical tests were conducted using a battery cycler (Biologic Instrument-WBCS3000, Gières, France) in a three-electrode configuration, where WO₃ and W@C thin films, platinum, and Ag/AgCl electrodes served as the working, counter, and reference electrodes, respectively. 1 M LiClO₄ + PC electrolyte was utilized as the lithium-ion source. To analyze the optical properties of the films in both the colored and bleached states, UV–Vis spectrophotometry (Model: S-3100, SCINCO) coupled with an electrochemical workstation (IVIUM Technologies, COMPACTSTAT, Capelle aan den IJssel, The Netherlands) was employed.

5. Results and Discussions

5.1. XRD Elucidation

The X-ray diffraction (XRD) pattern presented in the Figure 2 compares the crystallographic features of single-layered WO₃ (red spectrum) and the bilayer W@C composite (blue spectrum). The red pattern corresponds to the pure WO₃ thin film, with diffraction peaks indexed to the orthorhombic phase of WO₃ (JCPDS No. 00-020-1324). The prominent peaks at 2θ ≈ 23.1°, 24.3°, 26.6°, 34.1°, 49.8°, and 55.4° are assigned to the (001), (020), (200), (021), (400), and (141) crystal planes of orthorhombic WO₃. The presence of sharp diffraction peaks confirms the crystalline nature of the deposited WO₃ film. In contrast, the blue spectrum represents the bilayer W@C composite, where additional peaks corresponding to cubic CoO appear alongside the WO₃ reflections. The CoO phase is identified by diffraction peaks at 2θ ≈ 36.5°, 42.4°, and 61.5°, indexed to the (111), (200), and (220) planes of cubic CoO (JCPDS No. 00-042-1300). The retention of WO₃ peaks in the bilayer structure confirms the stability of the orthorhombic WO₃ phase, while the CoO peaks indicate the successful incorporation of CoO in the bilayer composite. The co-existence of orthorhombic WO₃ and cubic CoO in the bilayer structure suggests possible interfacial interactions that could enhance EC performance [28,29].

5.2. XPS Analysis

XPS analysis was employed to assess the stoichiometry, surface composition, and chemical states of the W@C bilayer composite thin film. The high-resolution W 4f core-level spectrum Figure 3a revealed two prominent peaks corresponding to the spin–orbit doublet of W 4f_7/2 and W 4f_5/2, with binding energies of 35.54 eV and 37.58 eV, respectively. The energy separation of approximately 2.08 eV between these peaks is characteristic of the W⁶⁺ oxidation state, confirming the presence of tungsten in its +6 oxidation state. Additionally, low-intensity peaks observed at 34.7 eV and 36.7 eV were assigned to the W⁵⁺ oxidation state. In contrast to the small peaks, the dominant W⁶⁺ state peaks reaffirm the presence of a significant quantity of tungsten species with the six-valence stoichiometry in the WO₃ material. Comparatively, the W 4f peaks exhibit a slight shift toward lower binding energy in the W@C composite thin film. This shift suggests an alteration in the local electronic environment of W due to possible interactions between WO₃ and the CoO species in the composite. Such a shift could arise from enhanced charge transfer between W and Co [30,31]. In Figure 3b, the O 1s spectrum for the W@C bilayer composite thin film deconvoluted into three peaks, a prominent peak at 528.8 eV, attributable to the W-O/Co-O bonds, which is typical for WO₃ and CoO species. This peak is also observed to shift slightly compared to the typical peak. This shift could be attributed to modifications in the oxygen bonding environment, possibly due to increased surface interactions within the composite structure. The secondary peak at 530.67 eV was assigned to oxygen-deficient regions, while the less intense peak at 532.8 eV was attributed to residual surface species, including hydroxyl groups, water, and C–O bonds, likely arising from exposure to ambient moisture during synthesis or handling. Finally, the Co 2p spectrum Figure 3c demonstrated the presence of Co in the composite, with two principal peaks at 779.9 eV and 795.2 eV corresponding to the Co 2p_3/2 and Co 2p_1/2 orbital states, respectively, along with characteristic satellite peaks. The atomic composition of the W@C bilayer composite was further analyzed using XPS, revealing atomic percentages of W = 15.75%, O = 72.02%, and Co = 12.25%. The W:O ratio is consistent with the expected stoichiometry of WO₃, confirming the presence of WO₃ with a dominant W⁶⁺ oxidation state. The Co content further supports the successful incorporation of the CoO layer in the composite structure. The presence of Co in the W@C composite suggests potential interactions with the WO₃ matrix, which may contribute to enhancing the electrochemical properties of the material. This observation provides compelling evidence for the successful formation of the W@C bilayer composite thin film and highlights its potential for improved functionality in EC applications [32,33].

5.3. Morphological and Elemental Composition Study

The FESEM images in Figure 4(a₁–b₃) provide a detailed view of the morphological characteristics of single-layered WO₃ and W@C bilayer composite thin films at different magnifications. These images are complemented by the EDX spectra in Figure 4(a₄–b₄), which offer valuable insights into the elemental composition of the films, aiding in the understanding of the structural and compositional differences between the two materials. In Figure 4(a₁–a₃), the FESEM image of single-layered WO₃ thin films reveals a densely packed nanogranular morphology. This morphology indicates uniform nucleation and growth of the WO₃ during the synthesis process, where the nanograins exhibit a smooth and homogeneous distribution [34]. The EDX spectrum shown in Figure 4(a₄) confirms the elemental composition of the single-layered WO₃ film, with tungsten (W) and oxygen (O) being the primary constituents, with atomic percentages of 56.14% and 43.86%, respectively. This confirms the successful formation of WO₃ [35]. On the other hand, the FESEM image in Figure 4(b₁–b₃) shows the morphology of the W@C bilayer composite thin films. These films exhibit a more complex and hierarchical structure compared to the W films. The W@C composite features highly interconnected granular and porous structures, with the morphology tending to form larger, well-defined spherical aggregates. The EDX spectrum in Figure 4(b₄) confirms the elemental composition of the W@C bilayer composite thin films, showing the presence of W (38.34%), O (25.12%), and cobalt (Co) (36.54%) in the composite structure. The compact nature of the nanograins in Figure 4(a₁–a₃) suggests a limited surface area and low porosity, which can restrict ion intercalation and diffusion, both of which are essential for efficient EC performance [36]. The reduced pore volume in this morphology may hinder the material’s ability to accommodate and transport ions during electrochemical cycling, thereby limiting the EC response, reversibility, and stability. In contrast, the W@C bilayer composite thin film in Figure 4(b₁–b₃) exhibits a more intricate, hierarchical structure with interconnected granular and porous regions, facilitating a significantly higher surface area and improved ion accessibility. The increased porosity of this morphology promotes enhanced ion diffusion and intercalation kinetics, which are crucial for optimizing the EC behavior, including faster response times and better cycling stability. As a result, the W@C bilayer composite thin film morphology is more favorable for EC applications due to its superior ion transport properties and greater electrochemical performance potential [10,21,37].

The cross-sectional FESEM image of the W@C film, presented in Figure 4c, provides clear evidence of the bilayer composite structure. The measured thicknesses are 350 nm for the WO₃ layer and 330 nm for the CoO layer, resulting in a total film thickness of 680 nm. The well-controlled layer thicknesses play a crucial role in enhancing the composite’s morphological and structural properties, which are expected to contribute significantly to its improved EC performance.

6. Electrochromic Analysis

The electrochemical behavior at the electrode–electrolyte interface was investigated through cyclic voltammetry (CV), emphasizing the crucial role of electrolyte ion diffusion in these systems. A detailed analysis of the CV responses of single-layer WO₃ and W@C bilayer composite thin-film electrodes was conducted using a three-electrode setup. The measurements were carried out in a 1 M LiClO₄ + PC aqueous electrolyte, employing an Ag/AgCl reference electrode. The scan rate varied from 10 to 100 mV/s within a potential window of +1 V to −1 V. Figure 5 a shows the combined CV curves at a scan rate of 10 mV/s, while Figure 5b,c depicts the CV profiles for single-layered WO₃ and W@C bilayer composite thin films over a scan rate range of 10–100 mV/s. The CV profiles exhibit a consistent electrochemical response, characterized by well-defined and broad redox peaks, indicative of substantial electrochemical activity and high reversibility. A clear linear correlation is observed between the enclosed CV area and the sweep rate, which can be attributed to the reduced ion diffusion path at higher scan rates, resulting in an increased current density [10]. The presence of symmetric redox peaks further confirms the excellent electrochemical reversibility of the W@C electrodes. Additionally, the progressive expansion of the hysteresis loop with increasing current density across different scan rates suggests enhanced electrochemical activity. This behavior reflects a more pronounced Li⁺ ion insertion/extraction process within the electrode material. The underlying mechanism can be associated with the reduction in the diffusion layer thickness at the electrode surface as the scan rate increases, thereby facilitating greater charge transport and leading to higher current density [38]. The W@C bilayer composite thin films exhibited higher peak currents compared to the single-layered WO₃ electrode, indicating superior electrochemical activity and, consequently, enhanced EC performance. In contrast, the single-layered WO₃ electrode demonstrated lower current densities, likely due to the limited availability of electroactive sites for Li⁺ diffusion, attributed to its dense and compact nanogranular morphology. The W@C bilayer films, however, featured a well-structured surface with highly interconnected granular and porous networks, forming larger spherical aggregates due to optimized deposition cycles. This structural arrangement facilitated efficient Li⁺ ion diffusion by creating a porous framework, thereby significantly improving the electrochemical performance compared to the single-layered WO₃ electrode [39]. The kinetics of Li⁺ ion insertion and extraction in single-layered WO₃ and W@C bilayer composite thin films were investigated by analyzing CV data, specifically examining the relationship between peak current and scan rate. Figure 5d demonstrates a linear dependence of redox peak currents on the square root of scan rates within the range of 10–100 mV/s for both single-layered WO₃ and W@C bilayer composite thin films. This linear trend in cathodic and anodic peak currents with increasing scan rates suggests a diffusion-controlled redox reaction. Based on this analysis, the diffusion coefficient of Li⁺ ions was calculated using the Randles–Sevcik Equation (1) at a scan rate of 10 mV/s [10]:

$${\mathrm{D}}^{{\displaystyle \frac{1}{2}}}={\displaystyle \frac{{\mathrm{i}}_{\mathrm{p}}}{2.69\times {10}^5\times {\mathrm{n}}^{3/2}\times \mathrm{A}\times \mathrm{C}\times {\mathsf{\vartheta }}^{1/2}}}$$

(1)

In this equation, i_p represents the peak current; n denotes the number of electrons involved in the electrochemical reaction (considered to be 1); A signifies the area of the working electrode (2 cm²); C refers to the concentration of the electrolyte; ϑ indicates the scan rate, and D is the diffusion coefficient (in cm²/s). Figure 5d shows that diffusion coefficients of single-layered WO₃ thin films show lower anodic (1.054 × 10⁻¹⁰) and cathodic (5.022 × 10⁻¹⁰) diffusion coefficients, while the W@C thin films exhibit significantly higher values, with an anodic diffusion coefficient of 4.296 × 10⁻¹⁰ and a cathodic diffusion coefficient of 14.089 × 10⁻¹⁰ at 10 mV/s. The enhanced diffusion coefficient for the W@C bilayer composite is attributed to its well-structured surface. The unique porous network of the W@C bilayer composite facilitates ion diffusion by shortening the diffusion path, thus providing more active sites for ion insertion/extraction. These structural features contribute to the increased diffusion coefficient observed for the W@C films [40].

The single-layered WO₃ and W@C bilayer composite thin films underwent quantification of Li⁺ ion intercalation and deintercalation using chronocoulometry (CC) under potential ranging from +1 to −1 V vs. Ag/AgCl, with each step lasting 40 s. Figure 6a,b show the charge versus time transient curves for the single-layered WO₃ and W@C bilayer composite thin films. During cathodic polarization, the films transition from a transparent to a colored state as charges are intercalated, while anodic polarization reverses this process, returning the films to their bleached state by deintercalating the charges. Electrochemical reversibility is a key parameter for assessing the EC performance of materials. To evaluate the EC reversibility of the single-layered WO₃ and W@C bilayer composite thin films, the intercalation charge (Q_i) and deintercalation charge (Q_di) were analyzed, as described by the following Equation (2) [21]:

$$\mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{d}\mathrm{i}}}{{\mathrm{Q}}_{\mathrm{i}}}}$$

(2)

Table 1. Evaluation of electrochromic measurements of single-layered WO₃ and W@C bilayer composite thin films.

Sample Name	Charge Intercalation $({\mathbf{Q}}_{\mathbf{i}}$) (C/cm2)	Charge Deintercalation $({\mathbf{Q}}_{\mathbf{d}\mathbf{i}}$) (C/cm2)	Reversibility (%)	Coloration Time (s) (TC)	Bleaching Time (s) (Tb)	Tb %	TC %	Optical Modulation (ΔT600nm%)	Optical Density(ΔOD)	Coloration Efficiency (cm2/C)
W	0.047	0.046	97.87%	11.2	6.02	90.3	12.03	78.27	2.01	85.53
W@C	0.051	0.050	98.03%	12.6	5.4	93.01	8.01	85.0	2.48	96.07

presents the estimated EC reversibility percentages for the WO₃ and W@C bilayer composite thin film. The W@C bilayer demonstrates the highest reversibility (98.03%), owing to their Q_i and Q_di values being higher than those of the single-layered WO₃ sample. The enhanced EC performance of the W@C films can be attributed to their porous surface morphology, which increases the surface area and facilitates improved ion transport and percolation into the film.

The EC performance of single-layered WO₃ and W@C bilayer composite thin films was further examined through in situ transmittance measurements, providing insight into their optical transitions between colored and bleached states. These spectra were recorded using a UV–Vis spectrophotometer coupled with an electrochemical workstation, covering a wavelength range of 350–1100 nm, with FTO glass as the reference baseline. Figure 6c,d illustrates the transmittance spectra of single-layered WO₃ and W@C bilayer films under applied potentials. When a −1 V bias was applied, the films exhibited a noticeable blue shift, attributed to the reduction in W⁶⁺ to W⁵⁺ and Co³⁺ to Co²⁺. Conversely, reversing the potential to +1 V restored the films to their bleached state, enabling precise optical characterization. Table 1 summarizes the in situ transmittance values for both the bleached (T_b%) and colored (T_c%) states, along with the optical modulation (ΔT = T_b − T_c) at 600 nm in the visible range. Initially, both electrodes displayed high transmittance in their bleached states. However, during coloration, notable differences emerged. The W@C electrode exhibited superior EC performance, achieving a lower transmittance of 8.01% in its colored state at 600 nm, compared to 12.03% for single-layered WO₃. These differences highlight the critical influence of surface morphology, structural composition, and electrochemical behavior on optical properties. The enhanced EC reversibility and coloration/bleaching efficiency of the W@C electrode underscore its superior EC performance [41,42].

The performance of EC materials is largely dependent on their coloration efficiency (CE), which measures the variation in optical density (ΔOD) per unit charge injected at a specific wavelength. A higher CE indicates superior EC behavior, as it signifies a greater change in transmittance (ΔT) for a given charge density. This key parameter can be calculated using the following Equation (3): [39]

$$\mathrm{CE}={\displaystyle \frac{\mathsf{\Delta }\mathrm{O}\mathrm{D}}{\left({\mathrm{Q}}_{\raisebox{1ex}{$\mathrm{i}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{A}$}\right.}\right)}}$$

(3)

The CC plot is utilized to determine Q_i/A, which signifies the charge injected per unit area of the electrode. This charge-dependent parameter is crucial as it quantifies the amount of charge within the sample that contributes to the change in ΔOD. It is calculated using the following Equation (4) [10]:

$$\mathsf{\Delta }\mathrm{O}\mathrm{D}=\mathrm{l}\mathrm{n}{\displaystyle \frac{\mathrm{T}\mathrm{b}}{\mathrm{T}\mathrm{c}}}$$

(4)

The transmittance values for the T_b and T_c states at 600 nm were used to calculate the ΔOD and CE for the single-layered WO₃ and W@C bilayer composite thin films. These values are summarized in Table 1. The CE calculations demonstrated that the W@C bilayer composite thin film achieved the highest coloration efficiency, approximately 96.07 cm²/C at 600 nm. This outstanding CE performance is primarily attributed to the uniform distribution of highly interconnected granular and porous structures. The morphology, which tends to form larger spherical aggregates, significantly enhances the active surface area and facilitates improved charge transfer processes. Furthermore, the synergistic interaction between the WO₃ and W@C bilayer structure plays a crucial role in further enhancing CE. In contrast, the lower CE observed in other single-layered WO₃ thin films can be linked to their suboptimal compact nanograin morphologies. Additionally, these films require a higher number of charge insertions during the coloring process, leading to only minor variations in optical density (ΔOD) [43].

The rapid switching of EC films between their colored and bleached states is essential for applications such as smart windows and displays. The coloration time refers to the duration required for the film to reach its colored state, while the bleaching time represents the period needed to return to a transparent state. Minimizing these transition times enhances the practicality of EC films by enabling swift modulation of their optical properties. In this study, the real-time transmittance of single-layered WO₃ and W@C bilayer thin films was monitored over a 40 s interval at 600 nm, as illustrated in Figure 7a,b, to evaluate their switching speeds. The coloration and bleaching times, defined as the time needed to reach 95% of the maximum optical contrast, serve as key indicators of device efficiency. The rapid coloration process is primarily attributed to the superior conductivity of tungsten bronze (Li_xWO₃) and cobalt bronze (Li_xCoO), which promotes efficient ion intercalation. In contrast, the sharp decline in current during the bleaching phase is associated with the transition of the material from a conductive to an insulating state. The results indicate that ion extraction occurs at a faster rate than ion insertion, leading to more efficient bleaching compared to coloration, owing to the enhanced conductivity of Li_xWO₃ and Li_xCoO during the bleaching process. Table 1 highlights the outstanding performance of the W@C bilayer thin film, which demonstrated an exceptionally fast coloration time of 12.6 s and an impressive bleaching time of 5.4 s, reinforcing its suitability for high-performance EC applications [10,21,37].

In smart window applications, the cycling stability of EC films is crucial for ensuring long-term performance. This stability allows the window to undergo repeated color changes while preserving its efficiency and functionality over time. The ability to control light and heat transmission effectively relies on the durability of the EC material. To assess the retention of EC properties, the thin films were analyzed using a UV–Vis spectrometer integrated with an electrochemical cyclic tester in a standard three-electrode setup. The cycling stability of in situ transmittance for single-layered WO₃ and W@C bilayer thin films is illustrated in Figure 7c,d. Each cycle, lasting 40 s at a wavelength of 600 nm, involved a complete transition between the colored and bleached states, providing insight into the films’ long-term EC performance [21]. The cycling stability of the EC films was assessed over 5000 s durations, with the single-layered WO₃ thin film shown in Figure 7c, while the W@C bilayer composite thin film depicted in Figure 7d. Remarkably, the W@C bilayer composite thin film exhibited only a 1.05% degradation in performance, underscoring its exceptional stability and durability in EC applications. This enhanced stability is attributed to the optimized surface morphology resulting from the bilayer structure of the two oxides, which strengthens the structural integrity of the film [37]. The W@C bilayer composite thin films feature a uniform distribution of highly interconnected granular and porous structures. This morphology, which tends to form larger spherical aggregates, enhances surface cohesion, minimizes ion trapping, and ensures strong adhesion to the substrate, thereby improving both the stability and EC performance. In contrast, the single-layered WO₃ thin film displayed a higher level of optical degradation, with a 72.03% reduction in transmittance after 5000 s of cycling. This significant deterioration in performance suggests extensive ion trapping within the film, which limits the availability of active sites for ion insertion and extraction. As a result, the film’s ability to maintain optical contrast during continuous coloration and bleaching cycles is compromised [44,45,46,47].

Recent studies have extensively explored the integration of transition metal oxides into WO₃ to enhance its EC performance. This approach addresses key limitations of individual metal oxides, including restricted durability, suboptimal color neutrality, and limited optical properties. The incorporation of mixed metal oxides provides a synergistic effect, optimizing EC efficiency and expanding functional capabilities.

Table 2. Comparative assessment of electrochromic parameters of W@C bilayer composite thin film with reported studies.

Se. No.	Material	Deposition Technique	Reversibility (%)	Optical Modulation (ΔT600nm%)	Coloration Efficiency (cm2/C)	Reference
1.	WO3–Nb2O5	Magnetron Sputtering	82.40	33	30.9	[7]
2.	WO3	Electrodeposition	87	68.89	154.93	[23]
3.	MnO2/WO3	Electrodeposition	-	55.6	51.7	[24]
4.	MnO2/Ni (OH)2	Electrodeposition	-	-	34	[25]
5.	TiO2/WO3	Hydrothermal	-	79.5	443.4	[26]
6.	TiO2–Co3O4	Hydrothermal	-	-	91	[27]
7.	WO3/(CoO)	Electrodeposition	98.03	85	96.07	This Work

compares the EC performance of various reported materials with the present study. The bilayered W@C film exhibited superior performance, achieving an optical modulation of 85.0% at 600 nm and a high coloration efficiency of 96.07 cm²/C. Notably, stability tests confirmed its durability, showing only a 1.7% degradation in optical contrast over 5000 s.

To further investigate the effect of different CoO electrodeposition cycles on the EC performance of WO₃, we conducted additional analyses for 10 and 30 electrodeposition cycles. The FESEM images, cyclic voltammetry, and transmittance data for these samples are presented in the Supplementary File (Figures S1–S3). These results provide insights into the influence of deposition cycles on the overall electrochromic behavior of W@C bilayer thin films.

7. W@C Based Electrochromic Device

The fabrication and evaluation of a prototype EC device are crucial for assessing the real-world applicability of EC materials, such as in smart window technologies. In this study, a large-scale EC device was developed using the W@C bilayer composite thin film, which was synthesized through the hydrothermal method. The device was constructed with a standard sandwich configuration, comprising the W@C EC active layer as the working electrode, an electrolyte layer, and a counter electrode. This design facilitates efficient ion transport and ensures optimal interaction between the electrodes and the electrolyte. Photographs of the device (Figure 8a) clearly demonstrate its impressive ability to transition between a transparent (bleached) state and a deep blue (colored) state upon the application of voltage. This rapid and distinct color change highlights the excellent EC activity of the W@C film, showing its capacity for quick and efficient switching. The in situ transmittance spectra of the EC device, measured over the 350–1100 nm wavelength range (Figure 8b), revealed significant optical modulation of 52.13% at 600 nm, showcasing the device’s superior ability to regulate light transmission in the visible spectrum. Notably, the difference in transmittance modulation between the W@C thin film and the EC device arises due to variations in ionic transport, optical path, and charge balancing. In the thin film setup, direct electrolyte contact enables efficient Li⁺ intercalation, leading to higher optical contrast. However, in the device, ion diffusion through the electrolyte layer introduces transport resistance, reducing optical contrast. Additionally, optical scattering at multiple interfaces and charge compensation limitations at the counter electrode further impact the overall ΔT%. Despite this, the device still demonstrates promising EC performance that is suitable for practical applications. This high modulation is particularly important for smart window applications, enabling effective control over light and heat transmission. Importantly, the EC performance of the device is comparable to that of traditional EC devices, demonstrating the W@C thin film’s effectiveness in practical device configurations. The bilayer composite structure of W@C thin films contributes significantly to the enhanced optical modulation capabilities of the device. Stability testing, which involved subjecting the device to repetitive coloration and bleaching cycles for 2000 s in the visible region (Figure 8c), showed that the device maintained stable optical modulation during the initial cycles. However, a slight reduction of 4.2% in optical modulation was observed after prolonged cycling, indicating some performance degradation. Despite this, the device continued to perform effectively, demonstrating the W@C thin film’s ability to retain functionality over extended periods. These results highlight the scalability and practical viability of the W@C thin film for real-world applications. The exceptional EC properties demonstrated by the device, even on a larger scale, confirm its suitability for use in energy-efficient smart window technologies [13,39].

8. Conclusions

This study thoroughly explored the EC performance of single-layered WO₃ and W@C bilayer composite films, revealing their outstanding potential for next-generation EC applications. The W@C bilayer composite thin film demonstrated remarkable electrochemical and optical characteristics, including an impressive optical modulation of 85.0% at 600 nm and a high coloration efficiency of 96.07 cm²/C. The film exhibited excellent long-term stability, with only a 1.7% degradation after extensive cycling, highlighting its exceptional durability for practical applications. Furthermore, the successful validation of the W@C -based EC device showcased the scalability of the material, achieving 52.13% optical modulation with reliable performance over time. These findings not only underscore the promising potential of W@C bilayer composite films in smart window technologies but also contribute significantly to the development of energy-efficient, sustainable systems. This work paves the way for the advancement of next-generation EC devices, fostering innovations that can meet the growing demand for energy-efficient solutions in modern applications.