Thermochromic Smart Windows Based on Gel–Sol Switchable Hydrogels with Thermal Stability, Bidirectional Temperature Responsiveness, and UV Shielding Performance

ABSTRACT Thermochromic smart windows hold significant promise for reducing building energy consumption via temperature‐responsive solar regulation. However, conventional systems often suffer from limited thermal stability, insufficient privacy protection, and poor UV shielding. Herein, we report a gel–sol switchable poly(N‐isopropylacrylamide)/gelatin (PNG) hydrogel‐based smart window with outstanding cyclic stability, bidirectional temperature responsiveness, and UV shielding. Leveraging the lower critical solution temperature behavior of poly(N‐isopropylacrylamide) (PNIPAM) and the reversible helix‐to‐coil transition of gelatin, PNG smart windows achieve dual‐mode optical switching with exceptional thermal stability. At elevated temperatures (>30°C), PNIPAM undergoes a gel‐to‐sol transition that renders the window opaque, with a high solar modulation efficiency of 70.4%. At low temperatures (<10°C), gelatin regains its triple‐helix structure, forming a dense hydrogen‐bonded network with PNIPAM for effective privacy protection by ultralow transmittance ( T lum = 2.2%). At ambient conditions (25°C), gelatin exhibits UV‐excited fluorescence, enabling high visible light transmittance ( T lum = 83.5%) while maintaining effective UV shielding (31.3% transmittance). Additionally, the PNG smart window demonstrates a fast response rate (10 s), high thermal emissivity (0.98), and superior aging resistance (continuous 200 h UV aging, 200 h heating at 60°C). This multifunctional hydrogel platform advances thermochromic smart window technology by unifying previously incompatible performance metrics for next‐generation building applications.