Hydroxypropyl cellulose thermochromic hydrogel with efficient passive radiative cooling and freezing resistance

Glycerol-modified antifreezing HPC l with LCST of 29 °C. • Record 86.1% visible modulation and 0.95 emissivity achieved. • 10.5 °C cooling effect with 77.5 W·m −2 theoretical power. • Ultra-low freezing point of −44.6 °C for cold climate use. • Stable performance maintained after 100 heating/cooling cycles. Thermochromic smart windows exhibit remarkable application advantages in the field of building energy conservation by virtue of their unique performance. Nevertheless, to meet the environmental requirements of specific scenarios, thermochromic smart windows must not only have optimal emissivity but also achieve a balanced match between high transmittance and solar reflectance. A novel hydrogel based on hydroxypropyl cellulose (HPC) was prepared in this study for high-performance radiative cooling smart windows. By adjusting the glycerol(Gly) content, the optimized HG₃₀ hydrogel achieves a low critical solution temperature (LCST) at approximately 29 °C. As a result, the HG smart window exhibits high emissivity (0.95), excellent visible light modulation capability (Δ T lum  = 86.1%), and a prominent solar transmittance modulation (Δ T sol  = 69.1%), along with a maximum transmittance of 91.01%. The HG smart window achieves a cooling effect of 10.5 °C relative to the empty double-layer glass cavity under direct sunlight, with a theoretical cooling power of 77.5 W/m 2 .After 100 high and low-temperature cycles and one month of exposure, both the transmittance and Δ T lum have no obvious changes. Moreover, the HG 30 smart window exhibits excellent antifreezing performance, ensuring sunlight transmission even under cold climate conditions. This study provides a significant reference for the development of high-performance and highly stable thermochromic smart windows, demonstrating broad application prospects in the field of building energy efficiency.