Hydroxypropyl Cellulose‐Based Thermochromic Hydrogels for Smart Passive Cooling

Abstract Solar heating through windows significantly increases thermal loads in buildings, vehicles, and greenhouses. In particular, overheating in parked vehicles under direct sunlight poses serious safety risks, with numerous reports linking high interior temperatures to heat‐related fatalities among children and pets. To address this challenge, a durable thermochromic hydrogel with a dual‐network structure of hydroxypropyl cellulose (HPC) and polyacrylamide (PAAM), enhanced by calcium chloride (CaCl 2 ) for tunable transition temperature is developed. Through ion chelation and hydrogen bonding, the hydrogel transitions between transparent and opaque states across a wide temperature range (15–42 °C), adapting to various environments. Encapsulated in acrylic sheets as smart windows, the hydrogel achieves passive cooling, reducing vehicle interior temperatures by up to 10 °C under direct sunlight. The material also exhibits excellent mechanical strength, water retention, and long‐term stability (400 thermal cycles), ensuring real‐world reliability. These results demonstrate the hydrogel's potential for scalable applications in vehicles, sustainable building facades, and greenhouse coverings, offering an energy‐efficient, eco‐friendly solution for thermal management. Unlike existing technologies requiring external power or complex fabrication, the hydrogel operates passively, making it a cost‐effective and sustainable alternative. This innovation addresses critical safety and energy challenges while advancing next‐generation energy‐efficient materials and global sustainability goals.