Synthesis And Electrochemical Capacitive Performance Of Hierarchically Ordered Porous Carbon Materials From Oligosilsesquioxane Precursors

Carbon-based supercapacitor is one of the most promising energy storage devices, which is characterized by short charge-discharge time, wide operating temperatures, long cycle life, safety and so on. However, compared with the secondary batteries, they have relatively lower energy density. Currently, regulating the pore structure of porous carbon electrode materials is the research focus in this field to increase the energy density of capacitors without decreasing the high power density. Many current studies have pointed out that if their pore size accurately matches the size of desolvated/bare electrolyte ions, those ultra-microporous carbons can provide the maximum electrochemical double-layer capacitance. However, the absence of the mesopores or macropores as ion diffusion/transfer paths in the ultra-microporous carbon structures results in significant capacitance fading at high current loads. Herein, we innovatively prepared hierarchically ordered micro-/mesoporous porous carbons by employing cubic polyhedral oligosilsesquioxane(POSS) intramolecular organic/inorganic hybrid structure as carbon precursor and self-template of producing uniform micropores, and block copolymer aggregates or colloidal crystals as soft or hard templates of producing ordered mesopores. Details are as follows:1. We demonstrated the synthesis of a new class of hierarchically porous carbons(HPCs) based on the self-assembly of cubic polyhedral oligosilsesquioxane(POSS) and amphiphilic block copolymers(BCPs). Owing to the precisely molecular-scale templating effect of POSS siloxane cages as well as the good assembly compatibility between the BCPs and the aminophenyl-functionalized POSS used, these POSS-derived HPCs possess both quite uniform micropores with the size of ~1 nm and highly ordered mesopores with the size of ~4 nm. They also present high specific surface area of over 2000 m2 g-1 and large pore volume of over 1.19 cm3 g-1. By simply varying the ratio of poly(ethylene oxide)/poly(propylene oxide) in the BCPs, the mesopore arrangement in the HPCs can be two-dimensionally hexagonal(p6m) or body-centered cubic(Im 3 m). Nitrogen functionalities can be introduced into these POSS-derived carbon materials since the OAPS used is a nitrogen-rich carbon precursor. These HPCs possess uniform micropores with the size close to that of the electrolyte ions, as a result, their maximum specific capacitance can reach ~163 F g-1 in ionic liquid electrolyte and ~216 F g-1 in aqueous 1.0 M H2SO4 electrolyte. The advantage of the HPCs over the strictly microporous carbons lies in their outstanding rate performance owing to dramatically reduced charge transfer resistance. The optimal sample demonstrated 94% and 97% of capacitance retention when the current density was increased from 0.25 to 10 A g-1 in ionic liquid and 1 M H2SO4, respectively.2. Owing to the limited types of the commercial Pluronic triblock copolymers and their relatively low molecular weight(generally, Mw < 15000), the sizes of the mesopores templated by the corresponding micelles are commonly difficult to be more than 5 nm. To investigate the influence of mesopore sizes on the electrochemical capacitive performance of the hierarchically porous carbon, we further prepared a series of hierarchically ordered micro-/mesoporous porous carbons with three-dimensionally interconnected mesopore structure and tunable pore size in the range from 10 to 40 nm by using silica colloidal crystals as templates and octa(aminophenyl)silsesquioxane(OAPS) as carbon precursor. When measured in ionic liquid electrolyte, it is found that, with the increase of mesoporous size, the rate performance of these HPCs obviously decline. A possible reason is that the thickness of mesoporous wall also increases with the increase of mesopore size. As a result, the diffusion resistance of electrolyte to the micropores on the mesopore wall also increases. The carbon sample with the minimum mesopore size(~14 nm) gives a large specific capacitance of 172 F g-1, and the capacitance could still remain 148 F g-1(86% of capacitance retention) when the current density was increased to 20 Ag-1.