Constructing highly anisotropic hexagonal boron nitride thermal networks via biomass-templating for robust phase change composites

Constructing continuous, highly oriented hexagonal boron nitride (h-BN) ceramic networks within organic phase change materials (PCMs) is a paramount strategy to overcome their intrinsic low thermal conductivity and leakage flaws. However, fabricating such anisotropic ceramic skeletons via conventional methods routinely suffers from high chemical processing costs, energetic calcination, or structural fragility under high filler loading. Herein, we report a robust and highly anisotropic h-BN thermal network successfully engineered via an eco-friendly water-based “biomass-templating” approach. Guided by a temperature gradient during directional freezing, upcycled coffee-waste-derived cellulose nanofibers (CNFs) serve as green structure-directing “rebars” to capture and vertically align flake-like h-BN ceramic “aggregates,” which are seamlessly welded into a stable 3D network by a sodium alginate “ionic cement”. This biomimetic processing strategy enables the composite to retain an exceptional organic PCM (n-eicosane) loading of 90 wt% while completely preventing high-temperature leakage. Benefiting from the continuous and low-thermal-resistance axial ceramic pathways, the optimized phase change composite delivers a remarkable axial thermal conductivity of 0.94 W m −1  K −1 (a ∼261% enhancement over the pure organic matrix) and a high latent heat of 182.06 J/g. Concurrently, the multi-scale chemical cross-linking endows the porous ceramic framework with an unprecedented compressive stress of 876.6 kPa, allowing it to support 1282 times its own weight without collapse, while maintaining robust near-superhydrophobicity (water contact angle of 146.85°). Practical outdoor miniature building model evaluations confirm that the composite panels exhibit superior dynamic thermal regulation, achieving an indoor peak temperature suppression of 8 °C and a 2-h thermal time lag. This work provides a facile, sustainable, and eco-friendly biomimetic templating strategy for designing highly oriented advanced ceramic networks for efficient thermal energy storage.