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. 2026 Jul 3;38(46):e22472. doi: 10.1002/adma.202522472

A Recyclable and Sustainable Hydroxypropyl Methylcellulose Electrolyte for Electrochromic Devices

Bang Yu 1, Fu‐Xing Zhao 1, Yi‐Jian Qian 1, Wei Peng 1, Meng‐Han Zhu 1, Chen Chen 1, Zong‐Ying Huang 1, Si‐Zhe Sheng 1, Zhen He 1, Jin‐Long Wang 1,✉
PMCID: PMC13486154  PMID: 42397014

ABSTRACT

Gel electrolytes, which supply compensating ions for electrochromic reactions and serve as conductive media, are widely used in electrochromic devices (ECDs). However, the complex preparation processes for gel electrolytes often involve the use of large amounts of organic solvents and hazardous or valuable reagents, limiting their widespread adoption in ECDs. This paper presents a straightforward method for fabricating large‐area hydroxypropyl methylcellulose (HPMC) gel electrolytes using a solution casting technique in an aqueous system. The addition of deep eutectic solvents (DES) and the incorporation of SiO2 nanoparticles significantly improve the conductivity of the gel electrolytes. The resulting HPMC/DES‐SiO2 gel electrolyte exhibits high transparency (over 92% transmittance), high ionic conductivity (7.55 × 10−4 S/cm), wide electrochemical window (±3 V), and outstanding thermal stability. The W18O49/NiO ECD assembled with HPMC/DES‐SiO2 gel electrolyte shows excellent cycling stability (maintained 94.8% of initial optical modulation after 1000 cycles) and can perform electrochromic response over a wide temperature range from −50°C to 200°C. Furthermore, the entire HPMC and up to 82.7% of LiTFSI and NMA can be recycled and reprocessed into new gel electrolytes, offering a promising solution for the future development of environmentally friendly and efficient ECDs.

Keywords: deep eutectic solvent, electrochromic device, hydroxypropyl methylcellulose, recyclable electrolyte

We propose a straightforward method for fabricating large‐area hydroxypropyl methylcellulose (HPMC) gel electrolytes using a solution casting technique in an aqueous system. The resulting gel electrolyte exhibits high transparency, high ionic conductivity, wide electrochemical window, and outstanding thermal stability. Furthermore, the entire HPMC and up to 82.7% of LiTFSI and NMA can be recycled and reprocessed into new gel electrolytes, offering a promising solution for the future development of environmentally friendly and efficient ECDs.

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1. Introduction

Electrochromic devices (ECDs) can reversibly regulate their optical properties (transmittance, absorption, and reflectance) by switching the electrochromic materials between the bleached and colored states under different operating voltages [1, 2, 3]. Therefore, they have a wide range of application prospects in the fields of intelligent building, low‐power display, and military camouflage [4, 5, 6, 7, 8, 9]. As reported, the high‐quality gel electrolyte that provides compensation ions for the electrochromic reaction as a conductive medium is a prerequisite for realizing ECDs with high visible light modulation, safety performance, and long service life [1, 10, 11, 12].

Currently, biomass‐based gel electrolytes, such as cellulose, gelatin, chitosan, and sodium alginate, have been developed and exhibit high conductivity, excellent flexibility, and stability at room temperature [13, 14, 15]. Recent advances have further extended their operational temperature range, particularly for energy storage applications. However, these studies focus on batteries and supercapacitors, where the performance metrics differ fundamentally from those required in ECDs [16, 17]. ECDs impose unique demands on gel electrolytes—namely, high visible transparency (>90%), a wide and symmetric electrochemical window (±2–3 V), compatibility with transparent conductive oxides and nanoscale electrochromic films, and uniform large‐area switching [17]. To date, biomass‐based gel electrolytes that simultaneously meet these stringent requirements across extreme temperatures remain scarce [18, 19, 20, 21]. It is worth noting that introducing deep eutectic solvents (DESs) based electrochromic electrolytes is a promising solution to overcome these challenges [22]. Essentially, DESs are eutectic mixtures of Lewis or Brønsted acids and bases with high design freedom, wide temperature range resistance, excellent ionic conductivity, and dissolution capabilities [19]. In particular, low volatility, low vapor pressure, and high thermal stability make them ideal candidates for gel electrolytes suitable for ECDs.

In this study, we used eco‐friendly, biodegradable hydroxypropyl methylcellulose (HPMC) as the polymer matrix, lithium bis(trifluoromethyl)sulfonylimide (LiTFSI) as the conductive component, and a small amount of poly(ethylene glycol) (PEG) as the plasticizer to prepare HPMC‐based gel electrolytes in an aqueous system. The comprehensive performance of the HPMC‐based gel electrolytes was further improved by introducing DES and SiO2 nanoparticles (NPs) into the system. The resulting HPMC/DES‐SiO2 gel electrolytes achieved over 92% visible light transmittance, 7.55 × 10−4 S/cm ionic conductivity, and a ±3 V electrochemical window, enabling the assembled W18O49/NiO ECDs with considerable electrochromic performance over a wide temperature range of −50°C to 200°C. Additionally, the HPMC/DES‐SiO2 gel electrolytes can be recycled with high performance for further applications.

2. Results and Discussion

2.1. Preparation and Structural Property of HPMC/DES‐SiO2 Electrolytes

HPMC is mainly produced from cotton fibers by the alkalization‐etherization process and has excellent film‐forming properties. As shown in Figure 1a, the widely available and low‐cost HPMC was first dissolved in water by heating and stirring, followed by the addition of a small amount of polyethylene glycol (PEG) as the plasticizer. At the same time, SiO2 NPs were added to the solution to increase the ionic conductivity of the gel electrolyte by providing more conductive channels for ions to some extent [23, 24]. Subsequently, LiTFSI was selected as the conductive component and N‐methylacetamide (NMA) as the DES, enabling the gel electrolyte to withstand wide temperature variations. This process is carried out with low‐speed stirring at room temperature to fully mix and remove air bubbles. Finally, the HPMC/DES‐SiO2 gel electrolyte with excellent visible light transmittance was obtained after the thermal evaporation of water. Figure 1b shows a schematic diagram of the interaction between DES and HPMC, and the FTIR spectra (Figure 1c and Figure S1) were applied to analyze the interactions between DES and the polymer components. The peaks at 1350 and 1200 cm−1 corresponded to the stretching vibrations of the S═O and C─F bonds of LiTFSI, respectively, which were also retained in the infrared spectra of HPMC/LiTFSI. The peak at 1658 cm−1 appearing in HPMC/DES corresponds to the stretching vibration of the C═O bond of NMA. The appearance of a weaker symmetric contraction vibration peak of Si─O─Si at 440 cm−1 for SiO2 in HPMC/DES‐SiO2 also proves the successful doping of SiO2 NPs. As shown in Figure 1d, the local zoom of the IR spectrum indicates that the C─H stretching bands of these gel electrolytes are slightly blue‐shifted from 2869 to 2879 cm−1, corresponding to the dissociation of the lithium salt. The chemical bonding between Lithium‐ion and the polymer can be demonstrated by the red‐shift of the C─O band of the polymer from 1060 to 1053 cm−1. The slight blue shift of the characteristic C─H stretching band of the polymer from 2877 to 2879 cm−1 in the spectra further suggests the interaction between LiTFSI and NMA. Figure 1e and Figure S2 are scanning electron microscopy (SEM) and energy dispersive x‐ray spectrometer (EDS) mapping of the HPMC/DES and HPMC/DES‐SiO2 gel electrolytes. The surface of HPMC/DES‐SiO2 and HPMC/DES is flat, and the distribution of C, O, F, S, and Si elements are uniform, indicating the homodisperse of the multi‐components of the gel electrolyte. The sparseness of the Si elements is due to the low content of SiO2 NPs. As shown in Figure 1f, Figure S3, and Table S1, to gain fundamental insight into the exceptionally wide‐temperature stability of the HPMC/DES‐SiO2 gel electrolyte, we performed density functional theory (DFT) calculations. The binding energies and interaction distances were evaluated for three representative subsystems—NMA‐NMA, LiTFSI‐NMA, and HPMC‐PEG‐LiTFSI‐NMA—at −50°C, 25°C, and 200°C. The HPMC‐PEG‐LiTFSI‐NMA system exhibits the strongest binding energies (−3.12 to −4.56 eV), which increase in magnitude with rising temperature, indicating that thermal activation enhances cooperative hydrogen bonding and ion‐dipole interactions among the polymer, plasticizer, lithium salt, and DES. Correspondingly, the shortest Li─O coordination distances reach 1.731 Å at 200°C, evidencing a tighter, more stabilized network under extreme heat. At −50°C, strong ion‐dipole interactions (Li─O distances as short as 2.094 Å) effectively suppress polymer chain crystallization, preserving flexibility. These molecular‐level interactions provide a fundamental understanding of how the HPMC/DES‐SiO2 electrolyte maintains its structural integrity and mechanical stability across the exceptionally wide temperature range from −50°C to 200°C.

FIGURE 1.

FIGURE 1

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Fabrication and characterization of the HPMC/DES‐SiO2 gel electrolyte. (a) Schematic illustration of a solution casting method based on an aqueous system to manufacture the HPMC/DES‐SiO2 gel electrolytes. (b) Schematic illustration of the structure of the HPMC/DES‐SiO2 gel electrolyte that is constructed using the HPMC polymer matrix filled with DES. (c) Fourier‐transform infrared (FTIR) spectra of the HPMC, HPMC/LiTFSI, HPMC/DES, and HPMC/DES‐SiO2. (d) FT‐IR spectra of the HPMC, HPMC/LiTFSI, HPMC/DES, and HPMC/DES‐SiO2 at the wavenumber of 2900–2800 cm−1 and 1100–1000 cm−1. (e) SEM‐EDS mapping images of the HPMC/DES‐SiO2 gel electrolyte. (f) Interaction between NMA‐NMA, LiTFSI‐NMA, and HPMC‐PEG‐LiTFSI‐NMA, as obtained from DFT calculations.

2.2. Optical, Electrochemical, and Thermal Stability Properties of the Gel Electrolyte

The electrolyte as the ion transport layer plays a critical role in boosting the performance of ECDs, so the ionic conductivity is an essential property of electrolytes. As shown in Figure S4, the ionic conductivity of the HPMC/LiTFSI gel electrolyte increased first and then decreased with the increase of LiTFSI concentration. When the concentration of LiTFSI was 0.5 mol/L, the ionic conductivity of HPMC/LiTFSI gel electrolyte was maximized to 1.29 × 10−4 S/cm, and the impedance was 167.4 Ω. The increase in ionic conductivity was due to the increase in dissociated Lithium‐ion with the addition of LiTFSI concentration. However, excessive LiTFSI limits the mobility of the polymer molecular chains, which hinders the ion migration and decreases ionic conductivity. In addition, the increase in salt concentration also affects the degree of ion dissociation, leading to the decrease in ionic conductivity. In order to further optimize the comprehensive performance of the electrolytes, different ratios of LiTFSI to NMA in HPMC/DES were obtained and labeled as HPMC/DES‐x (x = 2, 4, 6, 8), where x represents the molar ratio of NMA to LiTFSI. The preparation process of HPMC/DES‐x was similar to the preparation of HPMC/LiTFSI. As shown in Figure S5, the DES formed by different NMA to LiTFSI molar ratios had no significant effect on the visible light transmittance and electrochemical stability of the prepared gel electrolytes.

As shown in Figure 2a and Figure S6, the gel electrolyte resistance gradually decreased and stabilized with the increase of the molar ratio of NMA and LiTFSI, and the ionic conductivity of HPMC/DES‐x first increased and then stabilized with the increase of x. The ionic conductivity of the HPMC/DES‐6 gel electrolyte was maximized to 4.9 × 10−4 S/cm with an impedance of 69 Ω. The increase in ionic conductivity of the gel electrolyte is attributed to the increasing dissociation of LiTFSI with the increase of NMA addition. On the other hand, NMA also weakened the interaction between the molecular chain of HPMC and lithium salt, so that more lithium ions were involved in the ionic transport process, thus increasing the ionic conductivity. However, when NMA reaches a certain proportion, the dissociation degree of Lithium‐ion reaches saturation, and no more Lithium‐ion can contribute to the ion conductivity. In addition, too much DES will weaken the interaction between HPMC molecular chains, decreasing the mechanical properties. As a result, the fracture stress of the HPMC/DES gel electrolyte decreased from 0.93 to 0.16 MPa, and the corresponding fracture strain decreased from 275% to 130% (Figure S7).

FIGURE 2.

FIGURE 2

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Characterizations for the HPMC/DES‐SiO2 gel electrolytes. (a) Conductivity and breaking stress of HPMC/DES gel electrolytes with different LiTFSI/NMA molar ratios. (b) Conductivity of HPMC/DES‐SiO2 gel electrolytes with different SiO2 NPs contents. (c) Transmittance of HPMC/DES‐SiO2 gel electrolytes with different SiO2 NPs contents. (d) Linear sweep voltammetry (LSV) of HPMC/DES‐SiO2 gel electrolyte at a scan rate of 10 mV·s−1 using stainless steel as working electrode and counter electrode. (e) TGA curves of the HPMC/LiTFSI, HPMC/DES, and HPMC/DES‐SiO2 gel electrolyte. (f) Comparison of the ionic conductivity and transmittance of the reported electrolytes. (g) Optical photographs demonstrating the general cohesiveness of our HPMC/DES‐SiO2 gel electrolytes toward substrates such as plastics, glass, resin, metal, timber, and ceramic. (h) Digital photograph of large‐area HPMC/DES‐SiO2 gel electrolyte.

Furthermore, the fracture stress of the gel electrolyte was enhanced from 0.096 to 1.31 MPa, and the corresponding tensile fracture strain was also improved from 158% to 220% by increasing the content of HPMC (Figure S8). The increase of HPMC leads to an increase in polymer chain entanglement in the system, which makes Lithium‐ion migration difficult, and the ionic conductivity decreases from the initial 6.30 × 10−4 to 1.04 × 10−4 S/cm. The HPMC content also affects the visible light transmittance of the HPMC/DES gel electrolyte, which decreased from 93.2% to 85.4% (Figure S9. Therefore, a comprehensive comparison of the performance of HPMC/DES gel electrolytes with different HPMC contents leads to the conclusion that the HPMC/DES gel electrolyte with 0.2 g HPMC has the best overall performance in ionic conductivity, mechanical properties, and visible light transmittance.

In order to further enhance the ionic conductivity, SiO2 NPs with different sizes and amounts were added to the HPMC‐based gel electrolytes. As shown in Figure 2b and Figure S10, the ionic conductance increases and then decreases with the increase of the mass fraction of SiO2 NPs. When the SiO2 NPs content was 0.04 wt.% relative to the HPMC mass, the ionic conductivity of the HPMC/DES‐SiO2 gel electrolyte was maximized to 7.55 × 10−4 S/cm, and the impedance was 52 Ω. The increase in ionic conductivity is due to the fact that the added SiO2 NPs decrease the crystallinity of the system, which generates more free ions. On the other hand, the oxygen atoms in SiO2 interact with the lithium ions in the system to form a lithium‐rich phase, which also provides a new channel for the transport of lithium ions, so the ionic conductivity increases with the addition of SiO2 NPs [23, 24]. The decrease of ionic conductivity with the excess addition of SiO2 NPs is due to their agglomeration phenomenon, which is unfavorable to the transmission of Lithium‐ion [25].

As shown in Figure 2c and Figure S11a, the HPMC/DES‐SiO2 gel electrolytes with different contents and sizes of SiO2 NPs possessed a visible light transmittance of more than 90%. Besides, the size of SiO2 NPs has also been studied with less effect on the ionic conductivity of the gel electrolyte, but fluctuating within a certain value as the maximum ionic conductivity is about 7.55 × 10−4 S/cm when the SiO2 NPs size is 17 nm (Figure S11b,c). As shown in Figure 2d and Figure S11d, the HPMC/DES‐SiO2 gel electrolyte has a wide electrochemical window of ±3.0 V, the same as HPMC/DES, proving that the addition of SiO2 NPs does not affect the electrochemical stability. As shown in Figure S12, in contrast to the electrolyte without DES, the DSC curve of HPMC/DES‐SiO2 gel electrolyte shows no glass transition, crystallization, or melting peaks between −50°C and 200°C. Figure 2e illustrates the thermal stability of the HPMC/DES‐SiO2, HPMC/DES, and HPMC/LiTFSI gel electrolytes. The introduction of DES leads to a certain enhancement of the thermal stability of the electrolyte film, which has 95% residual weight at 235°C, and NMA can weakened the interaction between the molecular chain of HPMC and lithium salt, so both the storage modulus (G') and the tensile modulus of the HPMC/DES were lower than those of the HPMC/LiTFSI (Figure S13). The introduction of SiO2 NPs also leads to a further enhancement of the thermal stability of the gel electrolyte, which still has 95% residual weight at 286°C. The enhancement of thermal stability, G′, and the tensile modulus is that the doped SiO2 NPs as the core can form an aggregation network, inducing the interaction enhancement between the polymer chains. As shown in Figure S14,G′ remains consistently higher than the loss modulus (G″) across the entire temperature range from −50°C to 200°C, confirming that the HPMC/DES‐SiO2 system maintains a stable gel‐like state without undergoing a gel‐to‐sol transition or freezing. This behavior is critical for electrochromic applications, as it ensures mechanical integrity and stable electrode‐electrolyte interfaces throughout the device's operational temperature window. At −50°C, the gel electrolyte exhibits a storage modulus of approximately 40.9 kPa and a loss modulus of 1.78 kPa (with G′ > G″), indicating that the material remains in a viscoelastic solid state without crystallization or embrittlement. This behavior is attributed to the DES, which effectively suppresses polymer chain crystallization and maintains segmental mobility even at extremely low temperatures. The absence of a glass transition or freezing peak in this temperature range (consistent with the DSC result) further confirms the low‐temperature flexibility of the electrolyte. At 200°C, both storage and loss moduli decrease compared to room temperature, but G′ remains greater than G″ throughout, indicating that the gel network remains intact and the material does not undergo thermal degradation or flow. Mechanical testing at −50°C, 25°C, and 200°C confirms that the HPMC/DES‑SiO2 gel electrolyte retains structural integrity without embrittlement at low temperature or degradation/flow at high temperature, exhibiting temperature‑dependent tensile behavior consistent with a polymer network reinforced by the DES and SiO2 nanoparticles. We posit that the DES contributes to the exceptional thermal stability and low‐temperature flexibility through a synergistic combination of plasticization and suppression of polymer chain crystallization/phase separation, maintaining its amorphous, gel‐like state without freezing or decomposition even at the extreme low temperature of −50°C or high temperature of 200°C. Figure 2f presents the comparison of ionic conductivity and visible light transmission of HPMC/DES‐SiO2 gel electrolytes with gel electrolytes prepared from other polymer matrices in related literature [14, 22, 26, 27, 28, 29, 30, 31, 32, 33, 34]. It can be seen that HPMC/DES‐SiO2 gel electrolytes have higher ionic conductivity or visible light transmittance than HPC, CA, LiSMC, HEC/CMC cellulose‐based gel electrolytes, PVDF‐HFP, PAN, PEG, and H‐MOF polymer matrix gel electrolytes. Figure 2g shows that HPMC/DES‐SiO2 gel electrolytes have excellent adhesion to plastics, glass, resins, metals, timber, and ceramics, and good flexibility can also be found by bending the gel electrolyte on PET and twisting the gel electrolyte (Figure S15). As shown in Figure 2h and Figure S16, a 14 cm × 25 cm HPMC/DES‐SiO2 gel electrolyte film was easily fabricated in the laboratory, which had very favorable visible light transmittance (more than 92%) and low haze (less than 10%), providing the possibility of large‐scale manufacturing of gel electrolyte.

2.3. Application in Electrochromic Devices (ECDs)

Figure 3a and Figure S17 show the schematic structure of the typical ECDs, in which W18O49 nanowires (NWs) are the electrochromic layer (thickness = 220 nm), HPMC/DES‐SiO2 is the electrolyte layer, and NiO is the ion storage layer (thickness = 170 nm). A constant bias from −1 to −3 V was applied to the ECD for 30 s, and its transmittance spectra from 400 to 800 nm were measured by a UV–vis–NIR spectrometer. As shown in Figure 3b, the ΔT of the device at the wavelength of 660 nm increased by about 10% when the applied bias increased by every 0.5 V, and the maximum modulation rate was 67.0% at 660 nm. The performance of the electrochromic device assembled with the HPMC/DES‐SiO2 electrolyte was investigated across a range of temperatures. As shown in Figure 3c, the cyclic voltammetry (CV) curves reveal that the peak potentials gradually decrease as the temperature increases from 30°C to 120°C, indicating that Li+ insertion occurs at a lower applied potential under thermally activated conditions. Figure 3d and Table S2 display the switching curves under a ±2 V bias across a temperature range of 30°C–120°C. A strong positive correlation between temperature and performance was observed: the optical modulation increased from 26.9% to 56.2%. The coloration time decreased significantly from 9.8 to 5.0 s, while the bleaching time exhibited minimal change. Furthermore, the coloration efficiency (CE) of the device exhibits a substantial temperature dependence, increasing from 72.9 cm2/C (30°C) to 122.3 cm2/C (120°C) (Figure 3e). This comprehensive enhancement—encompassing modulation depth, switching speed, and efficiency—can be attributed to the thermally increased ionic conductivity of the electrolyte. The accelerated ion mobility facilitates more efficient charge transfer kinetics at the electrode/electrolyte interface during Li+ insertion/extraction, thereby boosting the overall electrochromic performance.

FIGURE 3.

FIGURE 3

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Fabrication of electrochromic devices with HPMC/DES‐SiO2 gel electrolytes. (a) Structural schematic diagram of an electrochromic device with HPMC/DES‐SiO2 gel electrolytes. (b) Visible light transmittance of electrochromic devices at different biases. (c) CV curves at a scan rate of 50 mV s−1 in the potential range from −3 to +3 V measured at varied temperatures. (d) Transmittance changes at λ = 660 nm for the W18O49/NiO‐based electrochromic device under a cyclic bias of ±2 V at 30°C, 60°C, 90°C, and 120°C, respectively. (e) Optical density as a function of the charge density at 660 nm (at 30°C, 60°C, 90°C, and 120°C). (f) Transmittance variation of W18O49/NiO‐based electrochromic devices at λ = 660 nm for 1000 cycles at ±2 V cycling bias. (g) Optical photographs of a monolithic, large‐area (10 cm × 10 cm) electrochromic device in the fully bleached (left) and deeply colored (right) states.

The cycling stability under different biases is presented in Figure 3f and Figure S18. Under ±2 V bias, the device demonstrates remarkable stability over 1000 cycles, with the modulation decreasing only slightly from 26.9% to 25.5%, retaining 94.8% of its initial value. While at ±3 V, near the electrochemical stability limit, the optical modulation of the device decays from 56.2% to 40.5% within 200 cycles. This gradual decline is primarily attributed to the prolonged operation at the critical potential limits of the electrochemical window, which leads to restricted kinetics for Li+ deintercalation during the bleaching process, consequently causing a progressive decrease in the transmittance of the bleached state [35]. To demonstrate the scalability of HPMC/DES‐SiO2 gel electrolyte and fabrication process, we have fabricated and tested a larger device with an area of 10 cm × 10 cm. A photograph of the large‐area device and its uniform coloration/bleaching performance are shown in Figure 3g and Movie S1. The device maintains excellent performance, confirming the feasibility for practical, large‐area applications. In addition, Prussian blue ECDs were assembled by HPMC/DES‐SiO2 gel electrolyte, which had a visible light modulation of more than 60% (Figure S19). The coloration and bleaching times were 3.4 and 6.2 s when the device was under a bias of −2 and +3 V from the in situ transmittance curve. Therefore, the HPMC/DES‐SiO2 gel electrolyte is universal for an electrochromic device.

The transparency, conductivity, and mechanical properties of gel electrolytes are the most critical performance metrics, which are all affected by temperature. However, most of the reported gel electrolytes are only studied at room temperature. When ECDs are inevitably exposed to harsh operating conditions, such as hot equatorial regions, cold climates, and high altitudes, the high/low temperature endurance of electrolyte will be required in these devices. As shown in Figure S20, the conductivity of the HPMC/DES‐SiO2 gel electrolyte becomes progressively higher as the temperature increases from 30°C to 100°C. The temperature‐dependent ionic conductivity of the HPMC/DES‐SiO2 electrolyte was determined from Arrhenius plots (Figure 4a). The high/low temperature performance of the fabricated W18O49/NiO‐based ECD was studied by placing the device in the center of a high/low temperature table at the set temperature for 30 min (Figure S21. Subsequently, cyclic constant bias of +2 and −2 V were applied to the ECD to observe the color change of the ECD at high and low temperatures. As shown in Figure 4b,c, and Movies S2 and S3, the ECD can achieve the electrochromic effect in the temperature of −50°C and 200°C, showing excellent high/low temperature endurance, indicating their application prospect in extreme environments. Figure 4d shows the comparison of the temperature range of resistance of HPMC/DES‐SiO2 gel electrolytes with gel electrolytes prepared from other polymer matrices in the related literature. It can be seen that the HPMC/DES‐SiO2 gel electrolytes have a wider temperature‐resistant range than PAM‐SC [36], IL [37], DES [22], PAMPS/PAAm [38], PZD [39], PAGE [40], PZHE [41], and OHE [42] gel electrolytes.

FIGURE 4.

FIGURE 4

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Wide range temperature endurance and recyclability of deep eutectic solvent (DES) type electrolytes. (a) Fitted curves of ionic conductivity of HPMC/DES‐SiO2 gel electrolyte with temperature change. (b) Optical photographs of the W18O49/NiO‐based electrochromic device in the colored state and bleached state at −50°C. (c) Optical photographs of the W18O49/NiO‐based electrochromic device in the colored state and bleached state at 200°C. (d) Comparison of working temperature ranges of the reported electrolytes. (e) Ionic conductivity of HPMC/DES‐SiO2 gel electrolytes with different numbers of recoveries. (f) Visible light transmittance of W18O49/NiO‐based electrochromic devices with recycled HPMC/DES‐SiO2 as electrolyte layer at different biases. (g) Optical photographs of electrochromic devices assembled with recycled HPMC/DES‐SiO2 gel electrolytes in the colored and bleached states.

2.4. Recyclability of HPMC/DES‐SiO2 Gel Electrolyte

In this work, the as‐prepared HPMC/DES‐SiO2 gel electrolytes could re‐dissolve in water and re‐form HPMC gel electrolyte film after drying. Although the loss of conductive component leads to a decrease in the conductivity of the gel electrolyte, most of the raw materials and all of the polymer matrix HPMC are expected to be recycled, enabling the sustainable production of high‐performance gel electrolytes. As shown in Figures S22 and S23, when only PEG was added during the gel electrolyte recovery for three times, the electrochemical window of the regenerated gel electrolyte showed almost no change, while the conductivity was reduced by 17.3% on average. As shown in Figure S24, the residual liquid of the recovered electrolyte had typical characteristic peaks of LiTFSI, NMA, and PEG. After the attempts, the electrolytes obtained by replenishing LiTFSI, NMA, and PEG in the recovery system had ionic conductivity approximating that of the initial electrolytes (Figure S25 and Table S3). The above results showed that the loss of LiTFSI and NMA was about 17.3% during the recovery process, and almost all of the HPMC was recovered. Therefore, the recovery of HPMC/DES‐SiO2 gel electrolytes can be achieved by adding the lost conductive components and plasticizers during the recycling process, and the recovered gel electrolytes still have high visible light transmittance (Figure S26).

Figure 4e shows that the HPMC/DES‐SiO2 gel electrolyte was recycled three times, and its ionic conductivity and electrochemical window had almost no change. In addition, we tested the corresponding electrochromic properties of the ECD that was assembled with the HPMC/DES‐SiO2 gel electrolyte obtained from the third recovery. Figure 4f shows the variation of transmittance of W18O49/NiO ECDs assembled using the recycled HPMC/DES‐SiO2 gel electrolyte under different bias, showing similar visible light modulation as that of ECDs assembled with the initial HPMC/DES‐SiO2 gel electrolyte. As shown in Figure S27, 1000 electrochromic cycles (working voltage ±2 V) were performed on the device, and the visible light modulation decreased from 26.5% to 25.1%, still having 94.7% of the initial modulation. Optical photographs of the corresponding colored and bleached states are shown in Figure 4g. As shown in Table S4, compared with existing DES for electrochromic applications, the HPMC/DES‐SiO2 electrolyte exhibits lower ionic conductivity and transmittance than liquid DES electrolytes, but outperforms solid DES electrolytes in these aspects. Notably, it demonstrates outstanding recyclability and high and low temperature tolerance.

3. CONCLUSION

In summary, we have developed a high‐performance, recyclable, and sustainable HPMC‐based gel electrolyte by the solution casting method in an aqueous system. This HPMC/DES‐SiO2 gel electrolyte exhibits high visible light transmittance, excellent chemical stability, and a wide electrochemical window, significantly enhancing the performance of the assembled W18O49/NiO ECDs. The incorporation of DES and SiO2 NPs endows the electrolyte with high‐temperature stability and low‐temperature resistance, enabling the ECDs to operate effectively across a broad temperature range. Notably, the recyclability of HPMC/DES‐SiO2 electrolyte offers substantial potential for the development of environmentally friendly ECDs. This innovation provides a new approach for reducing costs and promoting sustainable development in the field. Overall, our study demonstrates a crucial pathway for fabricating sustainable, biomass‐based electrolytes, advancing the commercial prospects of ECDs in applications such as building energy efficiency.

4. Experimental Section

4.1. Materials

Tetraethyl orthosilicate (TEOS) was purchased from J&K Scientific. L‐arginine, N‐Methylacetamide (NMA), and Nickel Sulphate (NiSO4·6H2O) were purchased from Shanghai Macklin Biochemical Co., Ltd. Ammonium hydroxide solution, Polyethylene glycol (PEG, Mn≈300), and Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) were purchased from Aladdin Reagent Co., Ltd. Tungsten hexachloride was provided by Heowns Biochem Technologies. Hydroxypropylmethylcellulose (HPMC) was purchased from Meryer Co., Ltd. All reagents were used without further purification.

4.2. Synthesis of SiO2 Nanoparticles (NPs)

SiO2 NPs with different sizes were synthesized by the seed regrowth method [43]. SiO2 seeds were synthesized in a weak base environment consisting of TEOS, water, and arginine. Then, SiO2 seeds were regrown in TEOS, ethanol, deionized water, and arginine emulsion to obtain SiO2 NPs with different sizes. Typically, 0.174 g of arginine was added to the flask with 174 mL of deionized water and mixed well, and then 10.41 g of TEOS was added to the mixture at 70°C and stirred vigorously for 24 h to obtain the SiO2 seed solution. The average diameter of the prepared SiO2 NPs was about 17 nm. Then 0.48 g of the seed solution was added to the mixture of 0.174 g of L‐arginine, 41.4 mL of deionized water, and 168.6 mL of ethanol, and finally different amounts of TEOS were added. The suspensions of SiO2 NPs with different sizes were obtained by stirring the above solution at 70°C for 24 h at 300 r/min (Figure S28).

4.3. Synthesis of HPMC/DES‐SiO2 Gel Electrolyte

As shown in Figure 1a, HPMC/DES‐SiO2 gel electrolytes were prepared by the solution casting method. Typically, an appropriate amount of SiO2 NPs was added to 18 mL of deionized water to obtain a homogenous solution by sonicating and then heated to 65°C. Subsequently, 2 g of PEG and 0.2 g of HPMC were added to the above solution. After complete dissolution, a DES solution composed of LiTFSI and NMA was added to the HPMC mixture, and stirred at 25°C with a stirring speed of 300 r/min for 90 min to obtain the gel electrolyte precursor. The gel electrolyte precursor was centrifuged to remove insoluble HPMC flocs, and the supernatant was spread flat in a Petri dish and dried in a vacuum oven for 12 h to obtain HPMC/DES‐SiO2 gel electrolyte. The gel electrolyte without SiO2 NPs was labeled as HPMC/DES, and the gel electrolyte without SiO2 NPs and NMA was labeled as HPMC/LiTFSI.

4.4. Recovery of HPMC/DES‐SiO2 Gel Electrolytes

To begin with, the lost PEG and DES solvents were added to 18 mL of deionized water and sonicated for 10 min to obtain a well‐dispersed mixture 1. And then the HPMC/DES‐SiO2 gel electrolyte to be recovered was sheared and poured into mixture 1, and stirred at 400 r/min for 4 h to obtain mixture 2. Finally, centrifuge mixture 2 at 8000 r/min for 5 min, pour it into a petri dish, and dry it in an oven at 60°C for 12 h to obtain the recycled HPMC/DES‐SiO2 gel electrolyte.

4.5. Synthesis of W18O49 NWs

Uniform W18O49 NWs were prepared by a solvothermal method (Figure S29) [3, 6, 7, 44]. Typically, 0.36 g of tungsten hexachloride was added into 480 mL of ethanol, followed by vigorous stirring until completely dissolved. The solution was transferred to a Teflon‐lined stainless‐steel autoclave and maintained at 180°C for 24 h. Then, it was centrifuged at 9000 r/min for 5 min, washed repeatedly with ethanol, and finally dispersed in ethanol.

4.6. Fabrication of NiO Films

NiO was synthesized on ITO glass by an in situ growth method [45]. Typically, 27.2 g of NiSO4·6H2O was dissolved in 270 mL of deionized water, and then a clean ITO glass was immersed in the solution with the non‐conductive side covered by tape. Thirty milliliters of ammonia was added to the above solution and reacted for 4 min, and the ITO glass was removed and washed off the residual liquid with water. Finally, the Glass/ITO/NiO film was annealed at a rate of 10°C/min to 350°C for 1 h (Figure S30).

4.7. Assembly of HPMC/DES‐SiO2 Gel Electrolytes of ECD

ITO/W18O49 NWs as working electrode and ITO/NiO as counter electrode were applied to prepare the ECDs. Typically, the prepared W18O49 NWs were sprayed onto ITO glass to obtain a Glass/ITO/W18O49 film. The surrounding of the Glass/ITO/NiO was covered with 3 M tape, and then a piece of HPMC/DES‐SiO2 gel electrolyte was placed onto it. Finally, Glass/ITO/W18O49 glass was covered above it to make full contact with the 3 M adhesive to obtain the encapsulated W18O49/NiO‐based ECD.

4.8. Sample Characterization

Infrared spectra of the samples were tested by a Bruker Vertex 70 V Fourier transform infrared (FT‐IR) spectrometer in ATR mode. The transmittance spectra were measured by a UV‐3600i Plus (Shimadzu Corporation) equipped with an integrating sphere (ISR‐1503), and all transmittance values reported were measured using air as the 100% reference. Mechanical properties were tested in the tensile mode by an electronic universal testing machine (Instron 68TM‐30 equipped with Instron AVE2 non‐contacting video extensometer) with a gauge length of 25 mm at a loading rate of 100 mm·min−1 , and three samples were tested for each experimental condition to obtain statistically reliable values. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured on a thermo‐gravimetric analyzer (Mettler Toledo TGA/DSC 1) with a heating rate of 10°C·min−1 under the nitrogen atmosphere. Impedance tests were performed with an electrochemical workstation (760E) at a scanning rate of 0.01 V·s−1. Transmission electron microscopy (TEM) imaging was performed on a Talos F200X G2 TEM operating at an accelerating voltage of 200 kV. The crystalline shapes of W18O49 NWs and NiO were analyzed by x‐ray diffraction patterns (XRD, Smartlab 3 kW). The surface morphology of the HPMC/DES gel electrolyte and HPMC/DES‐SiO2 gel electrolyte was measured by a scanning electron microscopy (SEM, Hitachi SU8230) at an accelerating voltage of 3 kV. The elemental analysis of the materials was also carried out with an energy dispersive x‐ray spectrometer (EDS) connected to the SEM.

Author Contributions

B.Y., F.‐X.Z., and J.‐L.W. conceived the idea and designed the experiments. J.‐L.W. supervised the research. B.Y. and F.‐X.Z. carried out the synthesis experiment and analysis. M.‐H.Z. and C.C. helped the electrochemical testing. B.Y., and S.‐Z.S. helped the UV‐VIS‐NIR spectra. Z.‐Y.H., and Y.‐J.Q. helped the TEM and SEM testing. W.P. helped the mechanical property testing and DFT calculation. F.‐X.Z., B.Y., J.‐L.W., and Z.H. analyzed the data and co‐wrote the manuscript. All authors analyzed and discussed the results.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73912‐sup‐0001‐SuppMat.docx.

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Supporting movie 1: adma73912‐sup‐0002‐MovieS1.mp4.

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Supporting movie 2: adma73912‐sup‐0003‐MovieS2.mp4.

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Supporting movie 3: adma73912‐sup‐0004‐MovieS3.mp4.

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Acknowledgements

We acknowledge the funding support from the National Natural Science Foundation of China (Grants 52373243, J.‐L.W., 22575112, J‐L.W., 224B2504, F.‐X.Z., and 52503317, S.‐Z.S.), Shenzhen Science and Technology Program (ZDSYS20220401161800001, J.‐L.W., KQTD20221101093559017, J.‐L.W., and JCYJ20220530113200002, J.‐L.W.), high level of special funds (G03050K002), and the Guangdong Provincial Key Laboratory of Sustainable Biomimetic Materials and Green Energy (2024B1212010003), Guangdong Innovative and Entrepreneurial Research Team Program (2023ZT10C027, J‐L.W.). The authors acknowledge the service of the Jiuzhang Calculation (https://ninecalc.com).

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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Associated Data

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Supplementary Materials

Supporting File: adma73912‐sup‐0001‐SuppMat.docx.

ADMA-38-e22472-s002.docx (7.3MB, docx)

Supporting movie 1: adma73912‐sup‐0002‐MovieS1.mp4.

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Supporting movie 2: adma73912‐sup‐0003‐MovieS2.mp4.

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Supporting movie 3: adma73912‐sup‐0004‐MovieS3.mp4.

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Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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