Self-assembly of polyhedral oligosilsesquioxane (POSS) into hierarchically ordered mesoporous carbons with uniform microporosity and nitrogen-doping for high performance supercapacitors

Polyhedral oligosilsesquioxanes (POSS), regarded as the smallest possible particles of silica, are used as carbon source and assembled into hierarchically porous carbon structures by a block copolymer-assisted method. The obtained carbon materials with high specific surface area of over 2000 m² g^-1 and large pore volume of over 1.19 cm³ g^-1 possess both quite uniform micropores with the size of ~1 nm and highly ordered mesopores with the size of ~4 nm, owing to the molecular-scale templating effect of POSS siloxane cages as well as the good assembly compatibility between the block copolymers and the aminophenyl-functionalized POSS used. The mesopore arrangement can be two-dimensionally hexagonal (p6m) or body-centered cubic (Im3-m) by simply adjusting different block copolymers. Nitrogen functionalities with a relatively high content (~4 wt%) can spontaneously be incorporated into those carbon materials. Benifiting from the uniform microporosity and the nitrogen doping, the specific capacitance of the POSS-derived hierarchically porous carbons can reach ~160 F g^-1 in ionic liquid electrolyte and ~210 F g^-1 in 1 M H₂SO₄ aqueous electrolyte, when measured at a current density of 0.25 A g^-1 in a symmetrical two-electrode cell. More importantly, the highly ordered mesopores can facilitate ions fast transportion to the fine micropores to achieve the excellent power performance. The hierarchial carbon sample with a hexagonal mesostructure and a high mesoporosity displays the best rate capability with 94% and 97% of capacitance retention in ionic liquid and 1 M H₂SO₄, respectively, with the current density range from 0.25 to 10 A g^-1. By combining self-assembly strategy with rich POSS chemistry, we believe that many other hierarchical hybrid materials or carbon materials with unique electrochemical properties can be synthesized.