Synthesis And Electrochemical Properties Of Porous Nitrogen-Doped Carbon-Based Nanostructures

Supercapacitors and lithium ion batteries have been widely applied in portable electronics, auxiliary power sources, electric vehicles and smart grids due to their unique energy storage mechanisms and merits. However, carbon based electrodes, resorted to physical energy storage generating from charge separation in the electric double layers formed at electrode/electrolyte interfaces, deliver a limited energy density. Metal oxides-based anodes for lithium ion batteries possess low electronic conductivity and sluggish lithium ion diffusion. The introduction of heteroatoms into carbon based materials for supercapacitors could induce pseudocapacitive effects and significantly enhance their energy density. The incorporations between metal oxide nanostructures and heteroatoms doped porous carbon frameworks not only effectively improve the electronic conductivity of metal oxides but also shorten the diffusion pathway of lithium ions and provide stress accommodation during cyclings. In this work, N and O co-doped activated carbon nanotubes, N and O co-doped porous carbon frameworks and N-doped porous carbon/ZnO composite polyhedrons were synthesized via the facile in situ carbonization of polypyrrole, polyaniline, and zeolitic imidazolate framework-8, respectively. When evaluated for supercapacitors and lithium ion batteries applications, all of them delivered a high specific capacity, excellent rate capability and cycling stability. The main contents are as follows:1) N and O co-doped activated carbon nanotubes:Nitrogen-and oxygen-containing activated carbon nanotubes have been successfully synthesized via a high temperature carbonization of polypyrrole (PPy) nanotubes followed by chemical activation. The first carbonization step ensures the formation of amorphous carbon nanotubes, which protect the nanotube morphology from destroying during the subsequent chemical activation process. The obtained activated carbon nanotubes with high nitrogen (17.94 wt.%) and oxygen contents (8.85 wt.%) show enlarged specific areas of 705.9 m2 g-1 compared to that of the pristine carbon nanotubes (212.4 m2 g-’). As expected, the activated carbon nanotubes exhibit enhanced capacitance properties, such as an enlarged specific capacity (384.9 F g-1 at 0.5 A g-1), excellent rate capability (201 F g-1 at 50 A g-1), and more stable cyclic stability (only 2.4% of specific capacitance retention after 500 cycles) due to its abundant micropores, high specific areas and abundant interfacial functional groups.2) N and O co-doped hierarchical porous carbon frameworks:Nitrogen- and oxygen-containing hierarchical porous carbon frameworks have been successfully synthesized by carbonization and simultaneous activation of polyaniline with KOH as activating agent. The obtained porous carbon frameworks with hierarchical pore structures show high specific surface area (2280.0 m2 g-1), large total pore volume (1.75 cm3 g-1), and abundant nitrogen (5.7 at.%) and oxygen (14.4 at.%) functional groups. As expected, the porous carbon frameworks synthesized with a KOH/polyaniline weight ratio of 4:1 exhibit improved capacitance properties, such as an enlarged specific capacity (428.1 F g-1 at 0.5 A g-1), excellent rate capability (299.7 F g-1 at 50 A g-1), and excellent cycling stability. The specific power density and energy density can reach 14275 W kg-and 18.6 Wh kg-1, respectively.3) N-doped porous carbon/ZnO composite polyhedrons:An in-situ calcination strategy has been developed for the synthesis of porous zinc oxide/N-doped carbon (ZnO/NC) polyhedrons, in which zeolitic imidazolate framework-8 (ZIF-8) serves as the precursor. The ZnO/NC polyhedrons with a hierarchical architecture possess a high specific surface area of 390.7 m2 g-1, high nitrogen content (19.99 at.%), and robust pore structures. The porous N-doped carbon frameworks can not only increase the electronic conductivity of ZnO, but also provide interior space for the fast diffusion of Li+ions and accommodate the volume variations during the charge and discharge cycles. When evaluated for lithium storage capacity, the hierarchical ZnO/NC polyhedrons exhibit high reversible discharge capacity (834.3 mAh g-1 at the initial low rate of 0.5 C,1 C= 978 mA g-1), superior rate performance (399.2 mAh g-1 at 5 C and 253.5 mAh g-1 at 10 C), and excellent cycling stability (677.9 mAh g-1 at 1 C after 400 cycles).