Thin Nacreous Films with Enhanced Mechanical Properties for Highly Efficient Durable Radiative Cooling

Abstract

Passive daytime radiative cooling (PDRC) is a promising path to tackle energy, environment and security issues originated from global warming. However, the contradiction between optical properties (especially desired high solar reflectivity) and other applicable performance (e.g. strength, modulus, durability and thickness) limits the practical applications of PDRC. Herein, we demonstrate a nacreous PDRC film integrating aramid nanofibers (ANFs) network with core-shell TiO2-coated mica microplatelet (Mica@TiO2) scatterers via a “Solvent exchange-Reprotonation” processing strategy for enhancing mechanical strength and durability without compromising optical performance. The slow but complete two-step protonation transition regulates the three-dimensional dendritic ANFs network with strong fibrillar joints, where overloaded scatterers (> 50 wt%) are stably grasped and anchored in alignment, thereby resulting in a high strength of ~ 112 MPa. Meanwhile, the strong backward scattering excited by multiple interfaces of core-shell Mica@TiO2 and interlamellar micropores guarantees a balanced reflectivity (~ 92%) and thickness (~ 25 µm). Notably, such design renders excellent environmental durability, including high temperature, UV radiation, water rinsing and scratch damage, to meet the realistic requirements. The practical PDRC cooling capability is further revealed by outdoor tests where attainable subambient temperature drops are ~ 3.35 ℃ for daytime and ~ 6.11 ℃ for nighttime, thus providing possibility for thermal protection of vehicles, mobile phones and cables exposed to direct sunlight. Consequently, both the cooling capacity which equals those of state-of-the-art PDRC designs and comprehensive outdoor-services performance, greatly push PDRC towards practical applications.