Synthesis Of Highly Graphitized Hierarchical Porous Carbon Materials For Supercapacitors

Activated carbon as the representative in conventional porous carbons has attracted a wide attention due to its large specific surface area. However, because its porosity is mainly narrow micropores in the range of 0.5-1.5 nm, which severely restricts the transport of electrolyte ions, active carbon electrode shows an intense decrease of capacitance at high current densities. Because the hierarchical porous structure can effectively shorten the diffusion distance of electrolyte ions and expedite the ion transport rate, resulting in a good capacitive performance even at high current densities, much effort of hierarchical porous structure has been developed to overcome the problem of activated carbon. Additionally, highly graphitized carbon materials have an excellent electrical conductivity, which can expedite electron transport rate, leading to a fast electron transport rate and a low internal resistance. Hence, based on the concerns above, we synthesized graphitized hierarchical porous carbon microspheres (HCMs) and graphitized hierarchical porous carbon with CNTs-embedded (C/CNTs) by using catalytic graphitization method combined KOH activation with sulfonated porous poly-divinylbenzene microspheres or sulfonated polystyrene with CNTs-embedded as the carbon precursor.Firstly, sulfonated porous poly-divinylbenzene microspheres were synthesized by traditional micro-suspension polymerization and sulfurization. Then, HCMs were synthesized by using catalytic graphitization method combined KOH activation with iron as the catalyst and hard template. From the N2 adsorption-desorption isotherms, it can be proved that the porosity and pore size distribution of HCMs was seriously impacted by the proportion of hard template and carbon precursor, and the KOH activation process can effectively increase the porosity and specific surface area of HCMs. The Raman spectrum and XRD patterns indicate that the two-step activation method destroyed the graphitization of HCMs, and the one-step activation method can not only avoid the destruction of graphitization but also create an excellent graphitization. Electrochemical analysis shows that the electrochemical performances of HCMs were also impacted by the proportion of hard template and carbon precursor, and the sample prepared by 1g sulfonated porous poly-divinylbenzene microspheres and 4g FeCl3 · 6H2O has the best rate capacitance, capacitance behavior and the smallest equivalent series resistance. In addition, because of the increase of porosity and specific surface area, the specific capacitance, capacitance behavior and rate capacitance of activated HCMs was effectively improved.The sulfonated polystyrene with CNTs-embedded were synthesized by an in-suit microemulsion polymerization, and C/CNTs were synthesized by using catalytic graphitization method combined KOH activation with iron as the catalyst. From the Raman spectrum, it can be found that C/CNTs have a better graphitization than porous carbons, indicating a better electrical conductivity of C/CNTs. The N2 adsorption-desorption isotherm shows that C/CNTs possess a hierarchical porous structure combining abundant macropores, mesopores and micropores. Electrochemical analysis indicate that C/CNTs present excellent electrochemical performances, having the specific capacitance of 304.1F g-1 (25 mV s-1), excellent rate capacitance (89% capacitance retention at 150A/g), high energy density (5.48Wh/kg) and high power density (28.8kW/kg). Additionally, compared with the activated CNTs C/CNTs have the higher specific capacitance and better capacitance behavior, and compared with porous carbons the rate capacitance of C/CNTs was improved.