One‐Step Fabrication of Antifreeze Cellulose‐Based Hydrogel With High Luminous Transmittance and Solar Modulation for Smart Windows

Thermochromic smart windows hold significant promise for building energy conservation; however, concurrently achieving high transmittance, high solar modulation, straightforward fabrication, and robust anti‐freezing performance—critical for real‐world deployment—remains a formidable challenge. Herein, a thermo‐responsive cellulose hydrogel smart window was developed via a simple one‐step stirring strategy using hydroxypropyl cellulose as the matrix, achieving the above performance simultaneously. Incorporation of glycerol as a cryoprotectant and NaCl as a transition modulator endows the system with robust subzero transparency and a precisely tunable lower critical solution temperature (LCST). The optimized 2‐mm‐thick device achieves an exceptionally high luminous transmittance of 90.2% below LCST and a superior solar modulation capacity (Δ T sol = 76.5%). More, the smart window shows a stable light modulation performance even after 500 thermal cycles. EnergyPlus simulations confirm considerable cooling energy savings across diverse Chinese climatic zones, with particularly pronounced benefits in tropical and subtropical regions. This work establishes a versatile strategy for designing thermoresponsive smart hydrogels, extending their applicability beyond energy‐saving smart windows to diverse fields including sensing, camouflage, and anti‐counterfeiting.