Ultra-anti-freezing and thermally stable hydrogel-derived liquid-based smart window for all-climate energy-efficient buildings
Thermochromic smart windows based on hydrogels suffer from inevitable freezing at subzero temperatures and dehydration at elevated temperatures, severely limiting their year-round applicability. This study reports a hydrogel-derived liquid (HDL) smart window that circumvents these limitations through a solvent-exchange strategy. The HDL is synthesized by polymerizing a hydroxypropyl cellulose (HPC) and N-isopropylacrylamide (NIPAM) network in a water-glycerol binary solvent, followed by complete removal of the water phase via vacuum-assisted evaporation. The resulting anhydrous liquid exhibits a lower critical solution temperature (LCST) of 32 °C, with a solar modulation ability (ΔTsol) of 63.2% and a luminous transmittance (Tlum) of 88.1% in the clear state. Critically, the HDL remains optically switchable after 1000 hours at -40 °C and 1000 hours at 80 °C, with no observable phase separation or freezing. The smart window prototype demonstrates a 12.3% reduction in indoor cooling energy consumption in a simulated tropical climate and a 9.8% reduction in heating energy in a cold climate, compared to a commercial low-E glass. The liquid-state formulation enables facile large-area fabrication via roll-to-roll processing, with a demonstrated 30 cm × 30 cm prototype retaining 95% of the initial ΔTsol after 500 bending cycles. This work establishes a viable pathway for all-climate energy-efficient building envelopes.