N-doped Porous Carbon Superstructures Derived From Hierarchical Assembly Of Polyimide

In the last decade,three-dimensional（3D）carbon superstructures（CSSs）constructed from the assembly of low dimensional segments such as carbon nanotubes or graphene sheets have attracted growing interests since these materials can both inherit the outstanding properties of their building blocks,and gain some unconventional perks,which thus render them appealing candidates for various electrochemical applications such as electrocatalysis and energy storage.Among the building blocks for CSSs,carbon nanosheets,represented by graphene,graphene oxide（GO）,and functionalized graphene,have become the focuses in this area since these two-dimensional（2D）materials can be easily manufactured into nanostructures with diversified dimensions,morphologies and functions.However,mainly obtained by the“top-down”exfoliation of graphite,the above carbon nanosheets have inhomogeneous shapes and sizes.Thermal treatment of polymers with aromatic backbones such as polypyrrole and polyaniline provide an alternative strategy towards carbon nanosheets.In this work,we demonstrate a facile template-free approach towards the construction of both polymer and carbon superstructures with polyimide（PI）as the key polymer precursor.The details of fabrication and application of CSSs are as follows:（1）PI superstructures were synthesized by a typical two-step method,but the use of solvothermal polymerization was novel,which was an effective way to achieve the self-assembly of polymer.By varying the polymerization conditions,the assembly of PI can be easily controlled,leading to the formation of various hierarchical structures（such as flower-like and lantern-shaped spheres）consisting of 2D nanosheets of PI.The possible reason for the formation of hierarchical PI structures were studied by changing the solvent,concentration used in synthesizing PI.A pressure-induced,concentration-dependant mechanism was proposed to explain the PI superstructures,which was proved to be reasonal by later experiments.Moreover,our approach could be further extended to synthesize PIs using different monomers with well-defined hierarchical structures,it is also believed that the protocol was suitable for other polymers that can be obtained through condensation polymerization.The excellent thermal stability of PI was confirmed by thermal gravimetric analysis.（2）With their high thermal stability,the PI derived hierarchical structures are successfully converted into nitrogen-doped porous carbons（NPCs）maintaining the superstructures by pyrolysis/activation treatment.Interestingly,the hierarchical structure shows profound impact on the surface areas of the resulting NPCs.Especially,the flower-like packing of the nanosheets in the NPC flower（NPC-F）render the sufficient exposure of the nanosheets during the activation process,leading to an ultrahigh specific surface area of 1375 m² g^-1.By systematically studying the effect of temperature and activation time to the performance of NPCs,we obtained that the optimal temperature and time.Benefiting from the large specific surface areas,hierarchical porous structure,high nitrogen doping content（3.46 wt%）and a self-supported structure,NPC-F manifests not only excellent catalytic activity for oxygen reduction reaction（ORR）but also can serve as a high stable electrode material for supercapacitors.