Electrochromic Adaptive Solar Heater and Sub‐Ambient Passive Daytime Radiative Cooler

ABSTRACT Heating, ventilation, and air‐conditioning (HVAC) systems are major contributors to global energy consumption, underscoring the urgent need for energy‐efficient building envelope technologies. Synergistic solar and radiative electrochromism offers a promising solution by leveraging the sun and outer space as sustainable thermodynamic resources. In this study, we demonstrate a dual‐mode electrochromic device that enables reversible, non‐volatile switching between solar heating and sub‐ambient radiative cooling, achieving annual energy savings of 73.7 MBtu and CO 2 emission savings of 4063 kg CO 2 in specific U.S. climate zones. The device can achieve true sub‐ambient passive daytime radiative cooling by 1°C in the cooling mode and raise it by 33°C above ambient in the heating mode during daytime operation, controlled by electrical voltage. The device employs electrodeposited lossy amorphous Cu‐Bi nanoclusters on defect‐activated monolayer graphene, yielding high optical contrast between cooling (solar absorptance α = 7.99%, thermal emittance ε = 93.58%) and heating ( α = 47.7%, ε = 20.14%) modes, with outstanding long‐term durability. This work paves the way for scalable, durable, and dynamic thermal regulation, advancing sustainable building technologies.