Effect of Annealing Temperature on Morphology and Electrochromic Performance of Electrodeposited WO₃ Thin Films

Abstract

The purpose of this study was to investigate the effect of annealing temperature on the structural, morphological, and electrochemical properties of tungsten trioxide (WO₃) films, fabricated via electrodeposition and annealed at 50 °C, 250 °C, and 450 °C. Structural analysis using X-ray diffraction (XRD) revealed temperature-induced modifications, transitioning from amorphous to crystalline phases. Morphological studies by field emission scanning electron microscopy (FESEM) demonstrated an increase in grain size with temperature (31 nm, 48 nm, and 53 nm) and the formation of cracks at higher annealing temperatures. Electrochemical characterization showed that the WO₃ film annealed at 250 °C exhibited superior redox activity, enhanced ion diffusion, and excellent reversibility. Optical studies highlighted its exceptional performance, with 79.35% optical modulation, a coloration efficiency of 97.91 cm²/C, and rapid switching times (9.8 s for coloration and 7.5 s for bleaching). Furthermore, long-term cycling tests confirmed minimal degradation after 5000 cycles, demonstrating durability. This work provides a comprehensive understanding of the annealing temperature’s impact on WO₃ films and underscores the novelty of achieving optimal electrochromic (EC) performance through temperature tuning, advancing the design of energy-efficient smart materials.

1. Introduction

In the pursuit of sustainable energy solutions, materials capable of dynamically controlling light transmission have become increasingly valuable for reducing energy consumption and improving energy efficiency in buildings, vehicles, and various electronic devices. EC materials, which can reversibly adjust their optical transmittance under an applied voltage, offer a promising pathway for such applications [1,2,3,4,5,6]. These materials have a profound impact on technologies like smart windows, low-power displays, and adaptive mirrors, enabling precise control over light and heat transfer, thereby contributing to energy savings. EC windows, for example, can minimize heating and cooling costs in buildings by modulating sunlight and reducing glare, while automotive and display industries benefit from the reduction in power consumption associated with light-modulating capabilities. Among the wide array of EC materials studied, WO₃ has gained substantial attention for its outstanding coloration efficiency, reversible optical modulation, and structural durability under repeated switching cycles [7,8,9,10,11,12,13].

The robust EC properties of WO₃ stem from its capacity to intercalate and de-intercalate ions, such as H⁺ or Li⁺, facilitating reversible transitions between transparent and colored states. This ionic exchange, combined with high chemical stability, makes it one of the most promising materials for commercial EC applications. When WO₃ films are deposited on transparent conductive substrates such as fluorine-doped tin oxide (FTO), they exhibit enhanced performance characteristics that include a broader optical modulation range and faster switching speeds, both of which are essential for the efficiency and responsiveness of EC devices [14,15,16,17,18,19,20]. In addition, WO₃ films deposited on FTO substrates are easily integrated into standard fabrication processes, which enhances their scalability and suitability for practical applications. However, achieving the highest EC performance requires precise control over the structural and morphological features of WO₃ films, as these factors directly influence critical properties such as ionic mobility, electronic conductivity, and overall EC efficiency.

Several studies have explored various techniques for enhancing the properties of WO₃ films. Zhang et al. investigated sol–gel WO₃ films annealed at different temperatures, revealing that crystallization occurred above 400 °C. XRD analysis showed improved crystallinity, while UV-Vis spectroscopy indicated a decrease in the optical band gap from 3.62 eV to 3.30 eV with higher annealing temperatures. These structural changes contributed to improved conductivity and EC performance [21]. Kadam et al. utilized a hydrothermal method to grow WO₃ nanorods on indium tin oxide (ITO)-coated glass, controlling the size and surface area by varying the concentration of propylene glycol (PG). Increasing PG concentration led to larger crystallites and a stable WO₃ film with a nanoparticle-like morphology [22]. He et al. modified WO₃ films by incorporating gold nanoparticles through spin coating, which enhanced conductivity and EC performance, particularly in an aqueous acid system [23,24].

Within these techniques, electrodeposition is particularly favored due to its cost-effectiveness, simplicity, and ability to produce uniform films with precise control over thickness and morphology [25,26,27]. This technique allows for fine-tuning of the film’s properties, which is essential for enhancing EC performance [28,29,30,31,32,33,34,35]. However, to achieve optimal EC properties, further structural tuning of electrodeposited WO₃ films is often necessary, as the as-deposited films typically exhibit an amorphous or semi-crystalline structure [36,37,38,39,40,41,42,43,44,45]. The crystallinity, porosity, and grain structure of the films are critical factors influencing EC performance. Specific structural arrangements promote efficient ion transport, stable cycling behavior, and desirable coloration–decoloration dynamics. Consequently, post-deposition modifications, such as annealing or controlled crystallization, are vital to refine the structure, increase surface area, and induce the crystalline phases known to enhance EC properties. These modifications facilitate the formation of efficient ionic channels and increased surface roughness, both of which are crucial for achieving faster switching speeds, improved coloration efficiency, and extended cycling durability [46,47,48,49].

Abareshi et al. reported electrodeposited WO₃ films on FTO glass using an aqueous peroxytungstic acid solution. The films were evaluated at annealing temperatures of 60 °C, 100 °C, 250 °C, and 400 °C. The WO₃ film annealed at 60 °C exhibited remarkable electrochromic performance, achieving high transmission modulation and a coloration efficiency of 64.1 cm² C⁻¹ at 638 nm [50]. Reddy G.V. et al. deposited WO₃ films on Corning glass and FTO substrates using RF magnetron sputtering and studied the effect of annealing temperatures (573 K, 673 K, and 773 K). SEM showed fewer cracks at 573 K and 673 K, with more cracks at 773 K. XRD confirmed the films were crystalline at all temperatures. The highest coloring efficiency (52.38 cm²/C) was at 573 K, while the lowest diffusion coefficient (1.7 × 10⁻¹⁴ cm²/C) was at 773 K, emphasizing the role of annealing in electrochromic performance [15]. Srivastava et al. electrodeposited WO₃ films for electrochromic windows, showing nanostructured, porous, and granular properties. Annealed at 100 °C, the films exhibited fast bleaching (8 s), high transmission modulation (68%), coloration efficiency (62 cm² C⁻¹), and ion storage (>20 mC cm⁻²) at λ = 632.8 nm. They also demonstrated rapid lithium diffusion (D_Li > 10⁻¹⁰ cm² s⁻¹) and high ion mobility (μ_Li > 10⁻⁷ cm² V⁻¹ s⁻¹), maintaining stable performance after 10⁴ cycles [18]. Yan Ng et al. reported that acid-treated WO₃·H₂O nanoplates on FTO glass converted to WO₃ after annealing at ≥200 °C. The nanoplates’ thickness decreased with increasing temperature. The best electrochromic performance was observed at 200 °C, with a coloration efficiency of 28.64 cm² C⁻¹, 58% optical modulation, good cycling stability, and fast ion insertion (9.6 s) and extraction (5.1 s) times [18].

Building upon the findings from previous studies, this research delves into the structural and morphological factors influencing the electrochromic (EC) behavior of WO₃ films electrodeposited on FTO substrates. Through a detailed and systematic analysis of the films’ properties, we aim to identify critical structural modifications that can enhance EC performance, thus enabling more precise control over WO₃ optical modulation, cycling stability, and coloration efficiency. By examining the complex relationship between microstructural features and electrochemical responses, this study offers valuable insights for optimizing WO₃ in energy-efficient applications. The results not only deepen our understanding of the structure–property correlations crucial for high-performance EC devices but also establish a strong foundation for the future design of advanced materials that can meet the sustainability demands of emerging smart technologies.

2. Experimental Section

2.1. Reagents and Materials

The materials used for the synthesis were of analytical grade and did not require any further purification. Fluorine-doped tin oxide (FTO) glass substrate, sourced from MTI. Co. Ltd., Seoul, Republic of Korea. Prior to deposition, the FTO glass was thoroughly cleaned in ethanol, acetone, and deionized (DI) water for 15 min each ultrasonication.

Sodium tungstate dihydrates (Na₂WO₄·2H₂O), 30% hydrogen peroxide (H₂O₂), nitric acid (HNO₃), lithium perchlorate (LiClO₄), propylene carbonate (PC), etc.

2.2. Synthesis of WO₃ Films

This study advances prior research by introducing specific modifications in the sol synthesis of WO₃ [21]. A 100 mL aqueous solution of Na₂WO₄·2H₂O was formulated at a concentration of 25 mM and mixed with double-distilled water (DDW), followed by stirring for 15 min to achieve uniformity. The addition of 0.5 mL of 30% hydrogen peroxide (H₂O₂) resulted in a deep yellow hue, signaling the formation of peroxo-tungstic acid (PTA). Nitric acid (HNO₃) was then incrementally added while maintaining the solution temperature at 45 °C, after which the PTA solution was allowed to cool to room temperature. The resulting PTA solutions, which maintained a constant Na₂WO₄ concentration of 25 mM, were utilized for the electrodeposition of WO₃ films at an applied potential of ±1 V. To investigate the impact of different annealing temperatures on the structural, compositional, and EC characteristics of the WO₃ films, 15 electrodeposition cycles were performed under each annealing condition. The films were categorized based on their annealing temperatures: W-50 °C, W-250 °C, and W-450 °C. This methodical variation in annealing temperatures provided valuable insights into the effects of thermal treatment on the properties of the electrodeposited WO₃ films. Figure 1 shows the visual depiction of the synthesis of WO₃ thin films at different annealing temperatures.

3. Material Characterization

The crystal structure of the samples was examined using X-ray diffraction (XRD; PAN Analytical) with Cu-Kα radiation. Field emission scanning electron microscopy (FE-SEM; S-4800 HITACHI, Ltd., Tokyo, Japan) was employed to analyze surface morphology. For electrochemical assessments, a three-electrode configuration was implemented with a battery cycler (Biologic Instrument-WBCS3000, Tokyo, Japan) electrochemical workstation. The EC optical transmittance was measured using a UV-Vis spectrophotometer (Model: S-3100, SCINCO, Seoul, Republic of Korea) connected to an electrochemical workstation (IVIUM Technologies, COMPACTSTAT, Eindhoven, The Netherlands). These measurements were conducted in a 1M LiClO₄ + PC aqueous electrolyte, where the WO₃ films acted as the working electrode, platinum served as the counter electrode, and Ag/AgCl functioned as the reference electrode.

4. Results and Discussion

4.1. XRD Elucidation

The XRD patterns of WO₃ films annealed at 50 °C, 250 °C, and 450 °C, as shown in Figure 2, reveal significant differences in crystallinity and phase composition, highlighting the impact of varying annealing temperatures on the structural properties of the films. Each pattern aligns with the monoclinic phase of WO₃, referenced by JCPDS No. 43-1035, which includes characteristic peaks at specific 2θ values such as 23.1°, 23.6°, 24.4°, 26.6°, 28.8°, and 34.1°, corresponding to planes like (002), (020), (200), (120), (112), and (202). These peaks become sharper and more intense as the annealing temperature rises, reflecting enhanced crystallinity and phase purity. At 50 °C (red pattern), the XRD peaks are weak and broad, indicating low crystallinity. This suggests a “nanocrystalline” or partially crystalline structure, where small, disordered domains form with only short-range atomic ordering. Peaks around 23.1° and 26.6° are faint, showing the beginning of the monoclinic WO₃ phase formation, but the structure remains too disordered to support effective EC performance. The limited crystallinity at this temperature restricts ion diffusion pathways, resulting in slow and inefficient EC response. At 250 °C (blue pattern), the peaks, especially at 23.1°, 23.6°, 24.4°, 26.6°, and 28.8°, become sharper and more defined, indicating a substantial increase in crystallinity and the formation of a stable monoclinic WO₃ phase. This intermediate crystallinity is ideal for electrochromism, as it provides an optimal balance between ion transport and structural stability. The peaks at 23.1° and 24.4° are particularly prominent at this temperature, reflecting ordered alignment along the (002) and (200) planes, which enhances ion diffusion and allows reversible ion insertion/extraction. This structure is neither fully dense nor fully amorphous, enabling fast switching times, high coloration efficiency, and excellent reversibility, which are essential for EC applications. At 450 °C (pink pattern), the XRD peaks are sharp and intense, especially at 23.1°, 23.6°, 24.4°, 26.6°, and 28.8°, confirming a highly crystalline, monoclinic WO₃ phase. The highly ordered crystal planes, such as (002) and (120), form a rigid framework that, while stable, restricts the pathways for ion movement, making the structure less ideal for EC cycling [45,46].

4.2. Morphological Study

The FESEM analysis of electrodeposited WO₃ films at different annealing temperatures reveals distinct morphological features associated with each temperature. At 50 °C, Figure 3(a1) and the corresponding cross-sectional image Figure 3(a2) (thickness ~ 180 nm) depict a microstructure characterized by small-sized nanogranules that are uniformly distributed across the film surface. This fine granularity suggests that the nanostructures are well-formed, with consistent spacing and size, indicating effective nucleation and growth processes at this lower temperature. When the annealing temperature is increased to 250 °C, the morphology shows a combination of smaller nanogranules merging to form larger, more uniform granules. The FESEM image Figure 3(b1) and its cross-sectional counterpart Figure 3(b2) (thickness ~ 240 nm) indicate a smoother surface with fewer gaps, demonstrating coalescence and improved structural organization within the film. The transition from smaller to larger granules highlights the changes in microstructure that occur with increased temperature. This morphology enhances structural integrity and improves charge transport, facilitating efficient ion mobility during EC processes.

At the highest annealing temperature of 450 °C (Figure 3(c1)), the FESEM analysis reveals significant changes, including the formation of cracks across the film surface. The cross-sectional image Figure 3(c2) shows a thickness of ~320 nm, but the surface morphology exhibits more agglomerated nanogranules, with many becoming nearly invisible, suggesting excessive grain growth and thermal stress. This morphology indicates that the structural integrity of the film is compromised, leading to a less favorable surface appearance compared to the lower annealing temperatures. The WO₃ films annealed at 250 °C demonstrate superior EC properties compared to those annealed at 50 °C and 450 °C due to their optimal microstructure. At 250 °C, the films exhibit a favorable balance of granule size and uniformity, characterized by a well-defined arrangement of larger granules formed from the coalescence of smaller nanostructures. In contrast, the films annealed at 50 °C have smaller, more uniformly distributed granules that may limit the overall electrochemical performance, while the films annealed at 450 °C suffer from crack formation and excessive compaction, which disrupts continuity and can hinder performance. FESEM analysis revealed a temperature-dependent increase in the average grain size of WO₃ films, measured as 31 nm, 48 nm, and 53 nm for films annealed at 50 °C, 250 °C, and 450 °C, respectively, showing a transition from small nanogranules to larger agglomerations accompanied by crack formation at higher temperatures. Thus, the 250 °C annealed films, with their ~240 nm thickness and superior granule morphology, offer the best combination of structural and electrochemical properties for effective electrochromism [14,21,28].

4.3. X-Ray Photoelectron Spectroscopy (XPS) Analysis

XPS analysis was utilized to determine the stoichiometry, surface composition, and chemical states of WO₃ thin films. The survey spectrum of the W-250 °C sample, shown in Figure 4a, confirmed the presence of tungsten and oxygen, along with trace amounts of carbon. The carbon signals were identified as unintentional contamination from the surrounding environment, emphasizing the need for a controlled atmosphere during the deposition process to minimize such interference. High-resolution core-level spectra for the W (4f) and O (1s) orbitals are depicted in Figure 4b and Figure 4c, respectively. Voigt fitting of the W4f core-level spectrum (Figure 4b) identified two distinct peaks, corresponding to the spin–orbit doublet of W4f _7/2 and W4f _5/2, with binding energies of 35.16 eV and 37.41 eV, respectively. The energy gap of approximately 2.25 eV between these peaks confirms the presence of the W⁶⁺ oxidation state, validating the successful formation of WO₃ films. In Figure 4c, Gaussian fitting of the O (1s) spectrum for the W-250 °C sample revealed a primary peak at 530.68 eV, attributed to the W-O bonds in the WO₃ framework. This peak reflects the robust interaction between tungsten and oxygen atoms, which is critical for the structural stability and performance of WO₃ films. A secondary peak, observed at 532.35 eV, is associated with residual adsorbed species, such as hydroxide groups, water, and C-O bonds, on the film surface. These surface species likely result from exposure to moisture during synthesis or handling, potentially affecting the films electrochemical behavior and overall functionality in applications like energy storage devices. Consequently, understanding the chemical states and surface characteristics is essential for improving the functional properties of WO₃ films [21,27,35].

5. Electrochromic Analysis

An in-depth evaluation of the CV characteristics of WO₃ electrodes was undertaken using a calibrated three-electrode electrochemical system, which provided optimal conditions for the acquisition of accurate and reproducible data. The experimental measurements were conducted across a potential range of +1 to −1 V, referencing an Ag/AgCl electrode to ensure reliability. For this analysis, a highly conductive 1 M LiClO₄ solution in PC was utilized, establishing a stable environment that enabled a thorough and stable examination of the electrode’s electrochemical behavior. The CV data for WO₃ samples synthesized with varying annealing temperatures (W-50 °C, W-250 °C, and W-450 °C) were meticulously captured at a consistent scan rate of 20 mV/s, effectively covering the designated potential range from +1 V to −1. Figure S1a presents the CV plot for the bare WO₃ electrode, which serves as a reference for evaluating the effects of annealing. Following this, Figure 5a displays the combined CV plots at a scan rate of 20 mV/s for bare WO₃, as well as for the electrodes annealed at 50 °C, 250 °C, and 450 °C. The profiles in Figure 5a underscore distinct electrochemical responses linked to each annealing temperature, providing invaluable insights into their respective behaviors. The CV profiles reveal a consistent pattern, showcasing prominent and well-defined redox peaks that signify substantial electrochemical activity and a high degree of reversibility. Furthermore, an outstanding linear correlation is evident between the area enclosed by the CV curves and the sweep rate. This correlation arises from increased scan rates, which effectively shorten the diffusion distance at the electrode surface, leading to a notable enhancement in current density. These findings highlight the dynamic electrochemical interactions occurring within the WO₃ electrodes, emphasizing the impact of different annealing temperatures on their performance. The observation indicates that increasing the sweep rates enhances the efficiency of electrochemical processes, primarily due to the decreased resistance to mass transport, allowing for more effective ion movement to and from the electrode surface [3,21].

Figure 5b–d demonstrate that the W-250 °C electrode displays a significantly higher redox peak current and a greater area under the CV curve, highlighting its enhanced electrochemical performance relative to the W-50 °C and W-450 °C electrodes. The W-250 °C electrode achieves an optimal balance of granule size and uniformity, characterized by a well-defined arrangement of larger granules resulting from the coalescence of smaller nanostructures. This configuration offers a plethora of reactive sites that facilitate effective charge diffusion. In contrast, the W-50 °C and W-450 °C films exhibit lower peak currents in their CV profiles, signaling a decline in electrochemical activity. The reduced performance is linked to the smaller size of their nanogranules and a more agglomerated, cracked surface structure, which restricts the availability of accessible reactive sites. Consequently, the compact nature of these structures impedes the interaction between the electrode and the electrolyte, diminishing chemical reactivity and overall electrochemical performance. This intricate interplay of factors ultimately results in a transition to a deep blue color during the EC process. Conversely, the subsequent oxidation process restores the electrode to its original transparent condition [39].

Subsequently, an intricate investigation into the charge transfer mechanism was undertaken based on the CVs to elucidate the current response mechanism. The plot of peak current density (i_p) against the square root of scan rate $({\vartheta }^{\frac{1}{2}})$ is depicted in Figure 5e, providing insights into the linear correlation between the redox current and scan rate. The determination of the diffusion coefficient of Li⁺ ions was accomplished using the Randles–Sevcik equation with i_p and ${\vartheta }^{\frac{1}{2}}$, conducted at a scan rate of 20 mV/s (1) [21].

$$D^{{\displaystyle \frac{1}{2}}}={\displaystyle \frac{i_p}{2.69\times {10}^5\times n^{3/2}\times A\times C\times {\vartheta }^{1/2}}}$$

(1)

In the equation, i_p represents the peak current, n denotes the number of electrons involved in the electrochemical reaction, A signifies the area of the working electrode (in cm²), C refers to the concentration of the electrolyte, ϑ indicates the scan rate, and D is the diffusion coefficient (in cm²/s). The resulting values for the cathodic and anodic diffusion coefficients of the WO₃ electrodes are summarized in

Table 1. Calculated Diffusion coefficient and evaluation of electrochromic measurements of WNi-1%, WNi-3%, and WNi-5% samples.

Sample Code	Diffusion Coefficient (cm2/s × 10−9)		$\mathbf{Q}\mathbf{i}$(C/cm2)	$\mathbf{Q}\mathbf{d}\mathbf{i}$(C/cm2)	Reversibility	tc (s)	tb (s)	Tb (%)	Tc (%)	ΔT600nm (%)	ΔOD	Coloration Efficiency (cm2/C)
	Reduction	Oxidation
W-50 °C	0.63	0.25	0.063	0.062	98.41%	8.4	4.2	91.30	18.80	72.50	1.58	50.15
W-250 °C	2.61	0.86	0.048	0.047	99.00%	9.8	7.5	87.65	8.30	79.35	2.35	97.91
W-450 °C	0.53	0.25	0.090	0.089	97.91%	8.8	3.7	91.30	50.72	40.58	0.58	12.88

, which clearly demonstrates that the W-250 °C electrode exhibits a higher diffusion coefficient (cathodic 2.61 × 10⁻⁹ cm²/s and anodic 0.86 × 10⁻⁹ cm²/s) when compared to the other reference electrodes. These findings provide compelling evidence that the EC performance of WO₃ can be significantly enhanced through the strategic modulation of annealing temperature, highlighting the critical role of thermal treatment in optimizing the electrochemical properties of the material [30].

This study examines how varying precursor concentrations influence the EC performance of WO₃ films, using in-situ transmittance spectra recorded in both the colored and bleached states. As shown in Figure 6a–c, the in-situ transmittance spectra were captured across a wavelength range of 300 to 1000 nm under an applied potential of ±1 V, with the transmittance outcomes summarized in Table 1. In the bleached state, the films display high transparency, while application of −1 V shifts them to a colored state. This coloration is induced by polaron absorption, resulting from the insertion of ions and electrons into the structure. Notably, Figure 6b highlights the W-250 °C sample’s outstanding optical modulation (ΔT) of 79.35% at 600 nm, with a bleached-state transmittance (T_b%) of 87.65% and a colored-state transmittance (T_c%) of 8.30%. This performance significantly surpasses the optical modulation of W-50 °C (ΔT = 72.50%) and W-450 °C (ΔT = 40.58%) [21]. The remarkable optical modulation exhibited by W-250 °C can be attributed to its ideal granule size and uniformity, featuring a structured array of larger granules formed by the coalescence of smaller nanostructures. This configuration shortens the path for ion movement and enhances the electrode–electrolyte interaction, facilitating more efficient ion insertion and extraction, which amplifies the coloration and bleaching effects. In contrast, the W-50 °C and W-450 °C samples show lower optical modulation efficiency, likely due to their less favorable surface morphologies [28,29].

The intercalation and deintercalation of Li⁺ ions in WO₃ electrodes were systematically examined over time using chronocoulometry (CC) analysis to gain insights into their EC behavior. Figure S1b presents the CC plot of the bare WO₃ electrode, serving as a reference to assess the electrochemical performance of the annealed samples. Following this, the CC plots for the three WO₃ samples are illustrated in Figure 7a for W-50 °C, Figure 7b for W-250 °C, and Figure 7c for W-450 °C. These plots detail the charge dynamics within each electrode immersed in a 1 M LiClO₄ in PC electrolyte, allowing for a comprehensive comparison of their electrochemical behaviors. During cathodic polarization, Li⁺ ions intercalate into the WO₃ matrix, introducing charges that reduce tungsten ions from W⁶⁺ to W⁵⁺, which induces the characteristic coloration effect of EC materials. When an anodic potential is applied, the Li⁺ ions are subsequently extracted from the WO₃ structure, and tungsten ions are restored to the W⁶⁺ state. This process returns the WO₃ film to its original transparent, bleached appearance, demonstrating the reversible EC behavior. This reversible transition between colored and bleached states is fundamental to WO_3′s EC functionality. A critical parameter in evaluating EC performance is the reversibility, which reflects the material’s ability to consistently switch between colored and bleached states over repeated cycles. Reversibility is quantified by the charge recovery ratio, defined as the ratio of charge removed during deintercalation (Q_di) to the charge inserted during intercalation (Q_i). This charge recovery ratio, calculated through a specific formula, provides a measure of the efficiency and stability of the EC process (2) [27].

$$Reversibility={\displaystyle \frac{Q_{di}}{Q_i}}\times 100$$

(2)

The Q_i and Q_di values, along with the calculated reversibility percentages, are summarized in Table 1. Among the evaluated electrodes, the W-250 °C sample demonstrates exceptional EC performance, achieving a high reversibility of 99%. This outstanding reversibility is attributed to the optimized surface morphology of the W-250 °C electrode, which promotes efficient ion movement and improves the structural stability required for consistent EC switching. In contrast, the W-50 °C and W-450 °C electrodes display significantly lower reversibility values, indicating a reduced ability to fully return to their bleached states during the deintercalation process. This reduction in reversibility likely stems from surface characteristics that hinder Li⁺ ion extraction, thus limiting the effectiveness of the bleaching process and compromising EC efficiency. Consequently, these findings underscore the critical role of fine-tuning annealing conditions, especially at 250 °C, to achieve enhanced EC functionality in WO₃ films [15,21].

Coloration efficiency (CE) is a critical metric in assessing EC materials, quantifying the effectiveness with which a material changes its optical density (ΔOD) in relation to the charge density applied. This parameter plays a significant role in enhancing both the energy efficiency and visual performance of EC devices, making it essential for their optimization and practical application. The mathematical formulation employed to calculate CE highlights its significance in optimizing device performance by ensuring an effective balance between minimal energy consumption and the desired optical shifts. This relationship is crucial for achieving maximum operational effectiveness in EC devices, allowing for both energy-efficient operation and optimal visual output (3) [1,8,9].

$$CE={\displaystyle \frac{∆OD}{\left(\raisebox{1ex}{$Q_i$}\!\left/ \!\raisebox{-1ex}{$A$}\right.\right)}}$$

(3)

This relationship links the change in optical density to the charge introduced per unit area of the electrode (Q_i/A) at a designated wavelength. The shift in optical density is directly dependent on the quantity of charge inserted, which is calculated from the transmittance values of the electrode in both its bleached and colored states, as represented by the following Equation (4) [35].

$$∆OD=ln\left({\displaystyle \frac{T_b}{T_c}}\right)$$

(4)

The transmittance values for the T_b and T_c at a specified visible wavelength were employed to determine the changes in ΔOD and CE for WO₃ films, as detailed in Table 1. Remarkably, the W-250 °C sample demonstrated the highest CE of 97.91 cm²/C at 600 nm among all the WO₃ films synthesized with varying annealing temperatures. This enhancement in coloration efficiency underscores the potential benefits of optimizing annealing temperature, which can lead to improved reaction kinetics and greater energy storage capabilities.

The remarkable optical modulation and enhanced CE of the W-250 °C electrode can be attributed to its finely tuned morphology, which strikes an ideal balance between granule size and uniformity. The presence of larger granules, formed through the coalescence of smaller nanostructures, creates an optimal active surface area that facilitates efficient charge transfer during electrochemical processes. This well-organized structure ensures that less charge is required to achieve significant changes in transmittance, leading to higher CE values. In contrast, the W-50 °C and W-450 °C samples exhibit diminished CE because their morphological characteristics necessitate greater charge insertion for coloration, resulting in smaller variations in ΔOD. Thus, the W-250 °C electrode’s superior performance highlights the critical role of morphology in enhancing the EC properties of annealed WO₃ films.

The rapid transition of EC films between colored and bleached states is crucial for their functionality in applications such as smart windows and displays. Coloration time refers to the duration needed to achieve a colored state, while bleaching time indicates the period required to revert to a clear state. Shortening these transition times enhances the practicality of EC films, enabling swift adjustments to their optical characteristics. In this study, the in situ transmittance of WO₃ films was measured and plotted in Figure 8a–c, assessing the switching times over a 40 s period at a wavelength of 600 nm. The coloration and bleaching times, defined as the time taken to reach 90% of the maximum optical contrast, are critical metrics for evaluating device efficiency. The observed fast coloration kinetics are attributed to the excellent conductivity of tungsten bronze (Li_xWO₃), which facilitates rapid ion intercalation. Conversely, the swift current decline during the bleaching phase results from the transition of the material from a conductive to an insulating state. These findings reveal that ion extraction occurs more swiftly than ion insertion, with bleaching consistently exhibiting superior performance compared to coloring due to the heightened conductivity of Li_xWO₃ during the bleaching process. Notably, Table 1 underscores the exceptional performance of the W-250 °C sample, which achieved a remarkably fast coloration time of 9.8 s and an equally impressive bleaching time of 7.5 s, demonstrating its potential for efficient EC applications [39].

Long-term stability in EC cycling is essential for the reliable operation of smart window applications, as it enables the window to effectively change color while maintaining efficiency in controlling light and heat transmission. In this study, the researchers evaluated the EC retention of WO₃ films by employing a combination of UV-Vis spectroscopy and an electrochemical cyclic tester in a three-electrode setup. The stability of in situ transmittance during cycling is depicted in Figure 9a–c, where each cycle lasts 40 s at a wavelength of 600 nm, alternating between colored (−1 V) and bleached (+1 V) states over a total duration of 5000 s. Among the samples, the W-250 °C film exhibited outstanding stability, experiencing only a minor decrease of 2.82% in ΔT after 5000 s. This exceptional performance can be attributed to its optimized surface morphology, which enhances stability, reduces ion trapping, and ensures robust adhesion to the substrate. In contrast, the W-50 °C and W-450 °C films showed more pronounced optical degradation, with declines of 5.66% and 8.47% in transmittance, respectively, after 5000 s. This diminished performance is associated with significant ion entrapment, which restricts access to active sites and adversely impacts overall transmittance [27]. These findings underscore the critical role of structural integrity in maintaining EC efficiency and the potential for optimizing film morphology to improve long-term stability in practical applications.

The results of this study, as detailed in the comparison Table S1 showcase groundbreaking advancements in the electrochromic performance of WO₃ thin films synthesized through the electrodeposition technique, particularly those annealed at 250 °C. These optimized films exhibit a highly controlled nanogranular morphology, resulting in outstanding electrochromic characteristics, including an impressive reversibility, optical modulation, and an exceptional coloration efficiency. Such remarkable performance underscores the superior potential of the electrodeposition method, with annealing playing a pivotal role in fine-tuning the electrochemical and optical properties. When compared to existing literature, these findings set a new benchmark in the field of WO₃-based electrochromic devices, offering unprecedented performance and long-term stability. This work not only advances the understanding of electrochromic materials but also paves the way for the development of more efficient and durable electrochromic supercapacitors, making a significant contribution to the field.

6. Conclusions

This study aimed to develop a cost-effective and scalable electrodeposition method for synthesizing WO₃ films on FTO glass, specifically designed to optimize their EC properties for large-scale applications. The focus was to investigate how different annealing temperatures influence the morphology and performance of WO₃ films, enhancing their suitability for advanced EC technologies. The main results demonstrate that annealing the WO₃ films at 250 °C yields a crystalline structure with ideal morphology, characterized by larger granules formed through the coalescence of smaller nanostructures. This morphology significantly enhances the electrochromic performance, achieving an optical modulation of 79.35%, near-perfect reversibility of 99.00%, and an impressive coloration efficiency of 97.91 cm²/C. These results underscore the crucial impact of annealing temperature in optimizing the electrochromic behavior of WO₃ films. The novelty of the results lies in the precise engineering of the film’s morphology through annealing, which not only boosts the electrochromic properties but also improves ion transport and electrolyte penetration, facilitating efficient transitions between colored and bleached states. The WO₃ films annealed at 250 °C emerge as highly promising candidates for smart window and display applications. Moreover, the electrodeposition method offers a cost-effective, scalable solution, making it highly suitable for the mass production of next-generation EC devices. This study highlights the potential of electrodeposited WO₃ films to revolutionize the commercial viability of electrochromic technologies.