An eco-friendly sodium-based thermoplastic starch solid electrolyte for energy-efficient and sustainable electrochromic devices

Abstract

This study presents an eco-friendly electrochromic device (ECD) utilizing a sodium-based thermoplastic starch solid electrolyte, in which carboxymethyl cellulose (CMC) enhances mechanical stability, sodium acetate serves as an ion source, glycerol acts as a plasticizer, and citric acid functions as a cross-linking agent. This system addresses the increasing demand for sustainable materials in energy-saving applications. The proposed sodium-based electrolyte offers a low-cost, abundant, and biodegradable alternative to conventional lithium-based systems. The ECD was fabricated using an “all-in-one” method, simplifying the assembly process with the composite electrolyte layer sandwiched between two ITO glass electrodes. The device demonstrated a fast coloration switching time of just 6 seconds at 90% modulation and 120 seconds at 99% modulation. Electrochemical analysis confirmed stable redox performance and efficient ion diffusion, while spectro-electrochemical measurements indicated a significant transmittance change of 58.5% at 520 nm. Moreover, the ECD exhibited excellent long-term stability, with minimal degradation in optical contrast of 6.0% over 80 hours of continuous operation. The self-bleaching time, without power, was recorded as 11.5 hours at 90% modulation and 16.5 hours at 99% modulation, showing efficient passive transparency recovery. These findings highlight the potential of sodium-based solid electrolytes for scalable, environmentally friendly and energy-saving ECDs.

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1. Introduction

Electrochromic devices (ECDs) have attracted significant attention and play an important role in buildings due to their ability to optimize energy efficiency.¹ With the characteristic of changing light transmittance under the influence of an electric field, ECDs allow control over the amount of light and heat entering the interior, helping to reduce the need for air conditioning and lighting.^2,3 Compared to photochromic^4–6 and thermochromic^7,8 glass technologies, which rely on environmental factors such as light and temperature, ECDs offer more flexible and precise control according to the user's preferences.⁹ The adoption of electrochromic windows as replacements for traditional windows, curtains in buildings, and especially shading materials in greenhouses can significantly reduce carbon emissions, driving the transition to sustainable and zero-emission buildings in the future.^10–12

Electrolytes are one of the essential components of any electrochromic device, serving the role of providing an ion transport channel and preventing short circuits. Unlike other electrochemical systems, electrochromic devices require the electrolyte to be more transparent and highly stable. Various types of electrolytes have been developed in liquid, solid, and gel forms, utilizing inorganic, organic, or hybrid materials.^13–17 Common electrolytes today are mainly based on lithium salts such as LiClO₄, LiPF₆, LiCF₃SO₃, and LiN(CF₃SO₂)₂, which are typically dissolved in organic solvents such as ethylene carbonate and propylene carbonate.¹⁸ However, these electrolytes have significant drawbacks, including the risk of leakage and environmental impact. As a result, researchers have turned to lithium salt electrolytes combined with gel matrices such as polyethylene oxide, polymethyl methacrylate, and polyvinyl chloride.^19–21 However, polymer-based gel electrolytes face several challenges, mainly related to cost, the need for strict control over synthesis processes, and their often toxic nature. Additionally, lithium resources are becoming scarce, and the extraction process poses many negative environmental impacts.²²

To address this issue, utilizing sodium acetate, a readily available and cost-effective resource, presents a promising solution. Thus, developing eco-friendly thermoplastic starch-based electrolytes that incorporate sodium ions offers significant potential for advancing sustainable electrochromic devices. The key benefits of these green electrolytes include their biodegradability, wide operating temperature range, and compatibility with redox processes, making them viable alternatives to conventional toxic chemical-based electrolytes in electrochromic applications.²³ Starch is frequently employed as a material due to its low cost and biodegradable properties;²⁴ however, films produced from starch tend to be rigid, hydrophilic, and often lack durability.^25–27 The incorporation of plasticizers like glycerol, sorbitol, or polyethylene glycol^28,29 can enhance flexibility.²⁸ To further improve the mechanical properties, non-toxic cross-linking agents like citric acid,³⁰ boric acid (cross-linking starch with poly(vinyl alcohol)), or glutaraldehyde (cross-linking poly(vinyl alcohol) with starch) can be used. Cellulose-based compounds are also frequently utilized to enhance the overall performance of composite films. Carboxymethyl cellulose (CMC), known for its high viscosity and non-toxic nature, offers stability³¹ and contributes to the biodegradability of the films.³² Pawlicka et al.³³ have investigated natural polymers like cellulose as electrolyte materials for electrochromic devices, and several other natural gel-based electrolytes have also been documented in earlier studies.^23,34,35

In this study, we report a strategy for developing an all-in-one ECD, which includes two pristine ITO glass plates and uses a thermoplastic starch-based electrolyte combined with sodium ions and electrochromic materials. This method provides convenience with simple and scalable processes. Users can easily sandwich the material between the two ITO glass plates, then heat and seal the device to successfully create an easy-to-make handmade ECD. Notably, the ECD exhibits high transparency and an optical contrast of 58.5% was achieved, along with a fast coloration response time and a contrast ratio of 99% at 20 s. The study demonstrated that the development and application of electrolytes derived from renewable resources such as starch and sodium not only contributes to the sustainable growth of the electrochromic industry but also help minimize environmental impact, especially in the global context of shifting toward greener technology solutions.

2. Experimental

2.1. Materials

4,4′-Bipyridyl (>98%) was supplied from Tokyo Chemical Industry Co., Ltd. (TCI). 2-Bromo ethanol (>98%) came from Macklin. Acetonitrile (ACS reagent, ≥99.5%), ethyl acetate (≥99.5%, ACS reagent) and ethanol (≥99.5%, ACS reagent) were purchased from Sigma Aldrich. Bio-based glycerol (>99%) was supplied from Duc Giang Chemicals Group. Citric acid (>99.5%) and sodium acetate (>99%) were obtained from Xilong Scientific Co. Commercial tapioca starch was obtained from Vietnam Tapioca Co., Ltd. Sodium carboxymethyl cellulose (Na-CMC, sodium content: 6.5–8.5%, viscosity: 300–800 mPa s) was purchased from Shanghai Zhanyun Chemical Co., Ltd. ITO glass (ITO film thickness ∼185 nm, resistivity < 6 Ω sq⁻¹, transmittance ≥ 84%) was supplied from South China Xiangcheng Technology Co., Ltd. (Shenzhen, China). All the chemicals were used without further purification.

2.2. Synthesis of bis(3-hydroxyethyl) viologen dibromide (HEV²⁺ 2Br⁻)

HEV²⁺ 2Br⁻ was prepared following the method described in our previous work.³⁶ Briefly, 10 mmol 4,4′-bipyridine and 30 mmol 2-bromoethanol were dissolved in 30 mL of acetonitrile and refluxed for 24 hours. After the reaction was complete, the mixture was allowed to cool to room temperature. The crude product was then precipitated using 200 mL of ethyl acetate, filtered, and rinsed with ethyl acetate. Recrystallization was performed using deionized water, and the final product, a white powder, was obtained after vacuum drying overnight. (¹H NMR (500 MHz, D₂O)): δ 9.32 (d, 4H), 8.8 (d, 4H), 5.3 (s, 2H), 4.8 (t, 4H), 3.9 (t, 4H).

2.3. Preparation of an EC layer based on thermal plastic starch (all-in-one)

Starch and CMC were dissolved in deionized water to prepare stock solutions with concentrations of 10 wt% for starch and 4 wt% for CMC. A mixture containing 2 g of 10 wt% starch solution, 5 g of 4 wt% CMC solution, 0.2 g of glycerol, and 0.2 g of citric acid was heated and stirred at 80 °C for 10 minutes. Subsequently, 0.64 g of sodium acetate was added to the mixture. Finally, 50 mg of HEV²⁺ 2Br⁻ was introduced into the prepared solution with vigorous stirring until a gel was formed. The resulting weight ratio of starch, CMC, glycerol, and citric acid was maintained at 1 : 1 : 1 : 1 (wt/wt).

2.4. Device fabrication

The fabricated EC layer was heated and applied onto the ITO substrate using the doctor blade coating technique, with a controlled thickness of 100 μm. A second ITO-coated substrate was immediately placed over the gel to form the ITO/EC layer/ITO structure of the device. The device featured an active area of 2.52 cm². The process for constructing EC layer-based devices (ECDs) is depicted in Fig. 1.

Fig. 1 (a) The structure of the ECD and schematic illustration of chemical interaction in the EC gel electrolyte; (b) absorption and (c) transmittance spectra of the ECD at varying applied potentials from 0 to −2.5 V. The inset images display the color changes of the ECDs as the applied potentials are adjusted from 0 to −2.5 V.

2.5. Coloring time and bleaching time

Coloring time and bleaching time were defined as the durations needed for the ECD to achieve 99% of its complete optical modulation when transitioning from the bleached state to the colored state and vice versa.

2.6. Electrochemical and spectroelectrochemical characterizations

Electrochemical measurements were conducted using a potentiostat/galvanostat (PalmSens4, PalmSens, Netherlands). Cyclic voltammetry was performed on the complete ECD at scan rates ranging from 100 to 500 mV s⁻¹. Spectroelectrochemical data were obtained by connecting the PalmSens4 device to a UV-vis spectrophotometer (Ocean Optics, USB2000) operating over a wavelength range of 190–900 nm. All ECD experiments were carried out at ambient temperature.

2.7. Diffusion coefficient

The diffusion coefficient (D) of the counterion (CH₃COO⁻) in the ECDs was determined using the Randles–Sevcik equation, expressed as follows:

i_p = kn^3/2AD^1/2cv^1/2 (1)

The current maximum value (i_p) of ECDs was directly proportional to the square root of the scan rate (v^1/2), with the slope represented as kn^3/2AD^1/2c.

2.8. Coloration efficiency

The coloration efficiency (CE) of ECDs was determined using the equation below:Here, Q represents the charge inserted, while T_b and T_c denote the transmittance values in the bleached and colored states of the ECDs, respectively.

3. Results and discussion

3.1. The structure and optical properties of the ECDs

Fig. 1a illustrates the structure of the ECD, with the all-in-one electrolyte placed between two transparent ITO glass electrodes. Starch and CMC within the electrolyte are cross-linked with citric acid, forming a thermoplastic composite structure. Citric acid was used as the cross-linking agent. Because of its polycarboxylic nature, esterification can occur between the carboxyl groups of citric acid and the hydroxyl groups of starch and CMC. Glycerol is used as a plasticizer because it can reduce internal hydrogen bonding between polymer chains, which in turn increases the spacing between the molecules.³⁷ HEV molecules are immobilized within the electrolyte through hydrogen bonds with the polymeric components of the electrolyte, leading to enhanced dispersion and physical immobilization within the gel network. This interaction effectively suppresses phase separation and crystallization of the viologen species, while also improving the solubility of HEV in the electrolyte. These combined effects not only promote reversible redox activity and high coloration efficiency, but also effectively inhibit the random migration of redox species, thereby enhancing the stability of the colored state.³⁶

The electrochromic performance of the ECD was analyzed, and the results are shown in Fig. 1b and c. When the applied voltage was reduced between 0 and −2.5 V, HEV in the ECD commences the reduction process, and there was an increased absorption at 520 nm as shown in Fig. 1b. The transmittance spectra and color change of the ECD were also analyzed and are presented in Fig. 1c. In the neutral state, the ECD exhibited approximately 70% transmittance across the entire visible range. Upon voltage application, the transmittance significantly declined across the visible spectrum, resulting in a dark violet color. The maximum change in transmittance (ΔT) at 520 nm was 58.5%, with a pulse duration of 20 seconds under different applied voltages.

3.2. Electrochemical and spectro-electrochemical properties of ECDs

Ionic conductivity and the electrochemical stability window are two critical parameters of the electrolyte layer. To optimize these, different concentrations of Na⁺ ions ranging from 0.5 M to 2 M were investigated and the optimal concentration was found to be 1 M (see Fig. S1 and S2). As shown in Fig. 2a and Table S1, the ionic conductivity of the ECD is 8.93 × 10⁻⁶ S cm⁻¹. The electrochemical characteristics of HEV were evaluated in a full ECD using cyclic voltammetry (CV) to assess its reversible redox behavior. Measurements were conducted within a potential range from −1.5 V to 1.5 V at scan rates from 100 mV s⁻¹ to 500 mV s⁻¹ (see Fig. S3). The cyclic voltammogram of HEV is displayed in Fig. 2b. During the first cycle, a reduction peak (I) at −0.24 V corresponds to the transition from HEV²⁺ to HEV⁺, while a subsequent peak (II) at −0.50 V indicates the conversion from HEV⁺˙ to HEV. In the reverse scan, oxidation peaks (I′ and II′) at 0.24 V and 0.50 V, respectively, signify the reoxidation of HEV to HEV⁺˙ and then to HEV²⁺, thus completing the redox cycle within the defined potential window. Based on the Randles–Sevcik equation, a linear correlation was observed between the cathodic peak current density (i_p) and the square root of the scan rate (ν^1/2), as shown in Fig. 2c. The high regression coefficient (R² = 0.998) suggests that the electrochemical process is diffusion-controlled and likely involves a single-electron transfer process. Furthermore, the diffusion coefficient of Na⁺ counterions was determined to be 7.88 × 10⁻¹⁵ cm² s⁻¹, indicating rapid ion diffusion. Efficient counterion mobility is crucial for achieving a rapid electrochromic response, in addition to the high conductivity of the electrochromic film and the composition of the electrolyte. In addition, Fig. 2d illustrates the transmittance change (%) of the electrochromic device (ECD) over time during a coloration and bleaching cycle. The device reaches the coloration state in approximately 6 seconds at 90% modulation. The transmittance rapidly decreased from 68.8% in the bleached state to 10.1% in the colored state. This indicates a fast response rate and effective switching capability between the device's states. After the device achieves the coloration state, the bleaching process occurs without the need for additional power, lasting about 11.5 hours at 90% modulation. The transmittance gradually increases, demonstrating that the bleaching process is slow yet stable without the requirement for a continuous power supply. This result emphasizes the capability of the device to maintain its colored state without constant power input, a critical feature for applications that demand energy efficiency and long-lasting color retention.

Fig. 2 (a) Nyquist plot of the ECD, with the inset showing the equivalent circuit; (b) cyclic voltammograms (CVs) of the ECD at a scan rate of 500 mV s⁻¹; (c) plot of peak current densities versus (scanrate)^1/2 for the ECD; (d) transmittance at 520 nm under a −2.5 V stimulus for up to 20 seconds in the ECD.

3.3. Colorimetry and coloration efficiency

The color space of the ECD can be accurately recorded through a colorimetry study. The CIE 1931 L*a*b* color space is used to analyze trends in lightness, chroma, and hue changes. In this system, L* represents the lightness level, ranging from black (0) to white (100), a* indicates the balance between red and green, and b* shows the balance between yellow and blue.⁴⁴ The L* values of the ECD start to decrease as higher voltages are applied (see Fig. 3a). Fig. 3b clearly shows the changes in chroma and hue. In the neutral state, the a* and b* values of the ECD are located in the second quadrant, indicating a transparent white color. As the a*b* coordinates move from the second quadrant to the fourth quadrant during the reduction process with increasing voltage, this trend aligns with the observed color transition (from transparent white to violet), recorded by the spectroelectrochemical method during the stepwise reduction process. The color space diagram visually illustrates the color change of the ECD, as shown in Fig. 3c, and this color variation aligns well with the trend observed in the a*b* plot. A high color difference (ΔE*ab) value of 58.34 was achieved between the bleaching state and the coloring state.

Fig. 3 L*a*b* values of the ECD at applied voltages from 0 V to −2.5 V. (a) Variation in the L* value of the ECD; (b) changes in the a* and b* values of the ECD; (c) color coordinates of the ECD in different states; (d) coloration efficiency (left Y-axis) and optical density (right Y-axis) as a function of charge density at a switching voltage of −2.5 V.

The plot of coloration efficiency (CE) as a function of charge insertion/extraction at an applied voltage of −2.5 V is depicted in Fig. 3d. The coloration efficiency (CE) of the electrochromic device (ECD) increased significantly, reaching a peak worth 324.14 cm² C⁻¹ at 20% of the full optical switch. However, it then decreased sharply to 185.21 cm² C⁻¹ at 90% of the full optical switch. The maximum CE value of the current ECD is 324.14 cm² C⁻¹, which is higher than that of other reported viologen-based ECDs (Table 1).

Table 1 Comparison of electrochromic performance properties for various viologen-based ECDs

Materials	Coloration voltage (V)	t c (s)	Memory time (h)	CE (cm^2 C^−1)	Ref.
HEV	−2.5	20	16.5	324.14	This work
PSV-2-MMA	−1.3	10	12	303	38
FBBDV-PB	−2.5	30	14	113.21	39
m-Polyviologen	−0.7	260	1.45	87.73	40
p-Polyviologen	−0.7	218	0.67	90.99	40
HERFV	−3	15	2.33	107.27	41
15.8-poly-viologen	−0.7	115	0.35	59.2	42
Viologen-ZnO NTs	−3	30	1	93.5	43

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3.4. Long-term stability of the ECD

As shown in Fig. 4a, the electrochromic device was tested by applying –2.5 V for 20 seconds to induce coloration, followed by an open-circuit condition (0 V) for 16.5 hours to allow bleaching, completing one full electrochromic cycle, repeated for 6 cycles. The ECD device exhibits high optical contrast. Table 1 provides a comparison of the performance of our device with that of previously reported viologen-based ECDs.^36–41 The maximum bleaching time for our device is 16.5 hours, which is longer than that of previously reported viologen-based ECDs. Fig. 4b shows the optical contrast retention of the device over time. Throughout more than 80 hours of operation, the optical contrast decreases only slightly. The decline was recorded at 1%, 2.2% and 6.0% after over 30 hours, 50 hours, and 80 hours, respectively. This indicates that the ECD has excellent optical contrast retention, with minimal degradation, demonstrating high durability and stable long-term performance.

Fig. 4 (a) Transmittance variation between the bleached and colored states of the ECD over 80 hours of operation and (b) retention of optical contrast in the ECD over 80 hours of operation.

Conclusions

In this study, we successfully developed an eco-friendly electrochromic device (ECD) by employing a sodium-based thermoplastic starch solid electrolyte. The device demonstrated a fast coloration switching time of 6 seconds and a high contrast ratio of 90%, with a maximum transmittance change of 58.5% at 520 nm within 20 seconds. Additionally, the ECD exhibited excellent long-term stability, maintaining over 94% of its optical contrast after over 80 h of continuous operation. Notably, the maximum self-bleaching time of the device, without the application of an external current, was recorded at 16.5 hours, showcasing its ability to revert to its transparent state passively. The device achieved a maximum CE of 324.14 cm² C⁻¹ at a switching voltage of −2.5 V. These results highlight the potential of using sodium-based electrolytes for energy-efficient and sustainable electrochromic devices, particularly in applications such as smart windows for buildings and greenhouses. Moreover, this study presents a simple yet effective strategy for utilizing renewable materials like starch and sodium to enhance the performance and environmental friendliness of electrochromic devices.

Author contributions

The study was conceptualized by Le Huy Thai, Cao Nu Thuy Linh, and Dr Le Hoang Sinh. Methodology was developed by Le Huy Thai, Cao Nu Thuy Linh, and Tran Thi Thanh Thanh. Investigation was conducted by Le Huy Thai, Cao Nu Thuy Linh, Tran Thi Thanh Thanh, and Phan Thi Minh Hang, while data curation was performed by Tran Thi Thanh Thanh and Phan Thi Minh Hang. Formal analysis was conducted by Cao Nu Thuy Linh and Tran Thi Thanh Thanh. The original draft was written by Le Huy Thai and Cao Nu Thuy Linh. During the revision process, Dr Tran Thanh Hoa contributed by conducting additional experiments, assisting in data interpretation, and supporting manuscript revisions. The review and editing of the final manuscript were completed by Dr Le Hoang Sinh. Dr Le Hoang Sinh also supervised the project, administered the research activities, and acquired funding for the study.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary Information contains additional figures, descriptive explanations, and detailed data tables that support and complement the findings presented in the main manuscript. See DOI: https://doi.org/10.1039/d5tc01990g

Acknowledgements

This work was funded by Vingroup Big Data Institute, Vingroup and supported by the Vingroup Innovation Foundation (VINIF), Vietnam, under project code VINIF.2020.DA20. This research was also funded by the Science and Technology Fund, managed by the University of Danang – VN-UK Institute for Research and Executive Education under project number VNUK-2025-Student 11.

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Footnote

† Le Huy Thai and Cao Nu Thuy Linh contributed equally to this work and are recognized as co-first authors.

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