Boosting Supercapacitor Performance of Graphene by Coupling with Nitrogen-Doped Hollow Carbon Frameworks

Rational design of one-dimensional skin-core multilayer structure for electrospun carbon nanofibers with bicontinuous electron/ion transport toward high-performance supercapacitors

The fast transport of electrons and ions within electrodes is crucial to the final electrochemical properties. Herein, we have developed a unique ultra-long one-dimensional (1D) skin-core multilayer structure based on electrospun carbon nanofibers mainly through a facile Stöber method combined with resorcinol-formaldehyde resin, which not only achieves bicontinuous electron/ion transport during the charge/discharge process, but also provides large surface area for ion adsorption. Particularly, controlling the number of active layers as well as regulating the active sites in layer both can obviously improve capacitive properties. Benefiting from the synergistic effects of the desirable architecture, such the rational-designed skin-core structural carbon nanofibers as supercapacitor electrode can deliver a high specific capacitance up to 255 F g-1 at 0.5 A g-1, favorable rate capability with 89% capacitance retention of initial capacitance at 8 A g-1, and excellent cycling stability with nearly 93% capacity retention after 10,000 cycles at 2 A g-1. Furthermore, the as-assembled symmetric supercapacitor devices also present a maximum energy density of 8.77 Wh kg-1 at 0.25 kW kg-1 and a maximum power density of 3.70 kW kg-1 at 6.74 Wh kg-1. Such skin-core carbon nanofibers provide an effective strategy to design high-performance supercapacitor electrode for the next-generation energy storage devices.

Nitrogen-Doped Hierarchical Porous Carbon Derived from Coal for High-Performance Supercapacitor

The surface properties and the hierarchical pore structure of carbon materials are important for their actual application in supercapacitors. It is important to pursue an integrated approach that is both easy and cost-effective but also challenging. Herein, coal-based hierarchical porous carbon with nitrogen doping was prepared by a simple dual template strategy using coal as the carbon precursor. The hierarchical pores were controlled by incorporating different target templates. Thanks to high conductivity, large electrochemically active surface area (483 m² g^-1), hierarchical porousness with appropriate micro-/mesoporous channels, and high surface nitrogen content (5.34%), the resulting porous carbon exhibits a high specific capacitance in a three-electrode system using KOH electrolytes, reaching 302 F g^-1 at 1 A g^-1 and 230 F g^-1 at 50 A g^-1 with a retention rate of 76%. At 250 W kg^-1, the symmetrical supercapacitor assembled at 6 M KOH shows a high energy density of 8.3 Wh kg^-1, and the stability of the cycling is smooth. The energy density of the symmetric supercapacitor assembled under ionic liquids was further increased to 48.3 Wh kg^-1 with a power output of 750 W kg^-1 when the operating voltage was increased to 3 V. This work expands the application of coal-based carbon materials in capacitive energy storage.

N-Doped Carbon Fibers Derived from Porous Wood Fibers Encapsulated in a Zeolitic Imidazolate Framework as an Electrode Material for Supercapacitors

Developing highly porous and conductive carbon electrodes is crucial for high-performance electrochemical double-layer capacitors. We provide a method for preparing supercapacitor electrode materials using zeolitic imidazolate framework-8 (ZIF-8)-coated wood fibers. The material has high nitrogen (N)-doping content and a specific surface area of 593.52 m² g^-1. When used as a supercapacitor electrode, the composite exhibits a high specific capacitance of 270.74 F g^-1, with an excellent capacitance retention rate of 98.4% after 10,000 cycles. The symmetrical supercapacitors (SSCs) with two carbon fiber electrodes (CWFZ2) showed a high power density of 2272.73 W kg^-1 (at an energy density of 2.46 W h kg^-1) and an energy density of 4.15 Wh kg^-1 (at a power density of 113.64 W kg^-1). Moreover, the SSCs maintained 81.21% of the initial capacitance after 10,000 cycles at a current density of 10 A g^-1, which proves that the SSCs have good cycle stability. The excellent capacitance performance is primarily attributed to the high conductivity and N source provided by the zeolite imidazole framework. Because of this carbon material's unique structural features and N-doping, our obtained CWFZ2 electrode material could be a candidate for high-performance supercapacitor electrode materials.