Sustainable All‐Biomass Radiative Coolers with Biomimetic Thorny Fiber for Enhanced Thermoelectric Power Generation

Biomass materials have garnered significant attention in radiative cooling due to their essential properties of infrared emissivity and environmental friendliness. Nevertheless, the solar scattering of raw biomass exhibits inherent limitations, restricting the development of all-biomass radiative cooling materials. Hence, inspired by the silica needle structures of Dendrocnide moroides, fully cellulose-based biomimetic thorny fibers (BTFs) are developed as high-performance radiative cooling materials, demonstrating an ordered architecture with the micrometer-sized fibers and pores (≈1-10 µm). Ordered fibers in the hierarchical pores are mainly formed by stacking cellulose nanofibers on the templates of hydrothermal-treated cellulose nanocrystals, accompanied with the Na⁺-mediated electrostatic self-assembly strategy and the extrusion-induced alignment provided by direct ink writing (DIW) 3D printing. The resulting BTFs exhibit an average reflectance of 91.0% in the visible spectrum and a high emissivity of 92.4% within the atmospheric window, enabling excellent daytime radiative cooling capability. Furthermore, the integration of DIW-printed BTFs into gradient-structured thermoelectric devices optimizes heat conduction, delivering an output power density of 7.61 W m^-2 at a temperature difference of ΔT = 30 °C by harvesting waste heat from electronic components. This study offers an innovative pathway for carbon-neutral cooling and sustainable energy applications.