Thermochromic Nanocellulose Films for Temperature-Adaptive Passive Cooling

Energy efficiency in habitation spaces is a pivotal topic for maintaining energy sufficiency, cutting climate impact, and facilitating economic savings; thus, there is a critical need for solutions aimed at tackling this problem. One viable approach involves complementing active cooling methods with powerless or passive cooling ones. Moreover, considerable scope remains for the development of passive radiative cooling solutions based on sustainable materials. Cellulose, characterized by its abundance, renewability, and biodegradability, emerges as a promising material for this purpose due to its notable radiative cooling potential exploiting the mid-infrared (MIR) atmospheric transmission window (8-13 μm). In this work, we propose the utilization of thermochromic (TC) materials in conjunction with cellulose nanofibrils (CNF) to confer temperature-dependent adaptivity to hybrid CNF films. We employ a concept where high reflection, coupled with MIR emission in the heated state, facilitates cooling, while high visible light absorption in the cold state allows heating, thus enabling adaptive thermal regulation. CNF films were doped with black-to-leuco TC particles, and a thin silver layer was optionally applied to the films. The films exhibited a rapid transition (within 1 s) in their optical properties at ∼22 °C, becoming transparent above the transition temperature. Visible range transmittance of all samples ranged from 60 to 90%, with pronounced absorption in the 8-13 μm range. The cooling potential of the films was measured at 1-4 °C without any Ag layer and ∼10 °C with a Ag layer. In outdoor field testing, a peak cooling value of 12 °C was achieved during bright sunshine, which is comparable to a commercial solar film. A simulation model was also built based on the experimental results. The concept presented in this study extends beyond applications as standalone films but has applicability also in glass coatings. Overall, this work opens the door for a novel application opportunity for green cellulose-based materials.