A Facile Strategy to Construct <scp>3D</scp> Nanofiber‐Assembled Freestanding Aerogel With Superlight and Efficient Thermal Insulation Properties

ABSTRACT A template‐free, freestanding three‐dimensional polyimide (PI) nanofiber aerogel was fabricated via the synergistic integration of electrospinning with gas foaming. Electrospun PI membranes were foamed at room temperature in NaBH 4 /methanol, wherein released H 2 bubbles uniformly expanded the fiber network into a hierarchically porous lamellar architecture. NaBH 4 concentration (0.1–0.9 M) and foaming time (5–25 min) were systematically varied to elucidate their influence on aerogel architecture and properties. This foaming process primarily induces physical reorganization of the nanofibers without compromising their intrinsic chemical structure. The resulting aerogels exhibit densities of 2.11–3.52 mg/cm 3 , porosity &gt; 95%, thermal conductivity 27.30–37.96 mW/m K (approaching that of air), and outstanding mechanical robustness and thermal stability, offering a lightweight, high‐performance thermal‐insulation platform for aerospace, cryogenic pipelines, and flexible electronics.