Study on Radiative Cooling Performance of Cellulose Nanofiber/Gelatin Aerogels Tuned by Fiber Aspect Ratio

Abstract The aspect ratio of cellulose nanofibers (CNFs) provides a single geometrical parameter for regulating the multiscale coupling and performance trade-off of bio-based radiative cooling aerogels. In this work, CNF/Gelatin composite aerogels were fabricated through aqueous assembly and freeze-drying by using CNFs with long, medium, and short aspect ratios as structural regulating units. Without chemical modification, hierarchical templating, or additional functional fillers, this model system enables direct evaluation of how fiber geometry governs dispersion state, network formation, pore-structure evolution, and the resulting optical, thermal, and mechanical properties. Increasing CNF aspect ratio strengthened fiber entanglement and network continuity, resulting in lower density, higher porosity, lower thermal conductivity, and improved dry-state compressive load-bearing and energy-dissipation behavior. However, the optical response did not simply increase monotonically with aspect ratio. The Medium-CNF/Gelatin aerogel exhibited the highest measured optical performance, with a solar reflectance of 89% and an infrared emissivity of 88%, which was associated with its smaller projected pore size and relatively narrower pore-size distribution. In contrast, the Long-CNF/Gelatin aerogel showed better thermal insulation and mechanical robustness because the longer fibrillar units promoted a more continuous, low-density, and load-bearing network. Outdoor tests further demonstrated sub-ambient cooling of up to 4.9 °C. These results reveal that CNF aspect ratio is not merely a descriptive size parameter but a geometrical lever for balancing projected pore morphology, heat transfer, and mechanical integrity in CNF/Gelatin radiative cooling aerogels. The two-component formulation enables the intrinsic geometry of cellulose-based building units to be comparatively evaluated without additional functional fillers or post-fabrication chemical modification.