A Recyclable and Sustainable Hydroxypropyl Methylcellulose Electrolyte for Electrochromic Devices

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.