For the last two decades, nanostructured carbon materials have attracted significant attention, as they exhibit unusual physicochemical properties different from their bulk counterpart. Nevertheless, agglomeration and re-stacking of carbon nanostructures have always limited optimal performance, with larger loading amount. To solve this issue, porous and compressible carbon-based sponges have been researched, but most of the previously suggested carbon foams were either not very porous, synthesized at high temperature heat treatment, and/or not applicable for carbon nanoparticles. In this work, we have successfully fabricated ultra-porous (porosity about 98-99%), polyimide-carbon nanoparticle (PI-C) composites by combining Ketjen black nanoparticles with PI short fibers, by simple freeze-drying and subsequent heat treatments at low temperature (240 degrees C). Based on the analysis, it has been discovered that the fabricated PI-C sponges not only exhibit highly robust mechanical properties but also retain low thermal conductivity even in comparison with pristine PI sponges. This work provides a milestone in fabricating a number of C-electrospun polymeric sponges by freeze drying and subsequent heat treatments, which are expected to be utilized in various fields of research. (C) 2019 Published by Elsevier Ltd.