Key issues facing electrospun carbon nanofibers in energy applications: on-going approaches and challenges

Electrospun carbon nanofibers (CNFs), with one-dimensional (1D) morphology, tunable size, mechanical flexibility, and functionalities by themselves and those that can be added onto them, have witnessed the intensive development and extensive applications in energy storage and conversion, such as supercapacitors, batteries, and fuel cells. However, conventional solid CNFs often suffer from a rather poor electrical conductivity and low specific surface area, compared with the graphene and carbon nanotube counterparts. A well-engineered porous structure in CNFs increases their surface areas and reactivity, but there is a delicate balance between the level and type of pores and mechanical robustness. In addition, CNFs by themselves often show unsatisfactory electrochemical performance in energy storage and conversion, where, to endow them with high and durable activity, one effective approach is to dope CNFs with certain heteroatoms. Up to now, various activation strategies have been proposed and some of them have demonstrated great success in addressing these key issues. In this review, we focus on the recent advances in the issue-oriented schemes for activating the electrospun CNFs in terms of enhancing the conductivity, modulating pore configuration, doping with heteroatoms, and reinforcing mechanical strength, in close reference to their applications in supercapacitors. The basic scientific principles involved in these activation processes and their effectiveness in boosting the electrochemical performance of CNFs are examined. Finally, some of the on-going challenges and future perspectives in engineering CNFs for better performance are highlighted.

Free-standing cross-linked activated carbon nanofibers with nitrogen functionality for high-performance supercapacitors

Flexible, heteroatoms-rich activated carbon nanofibers with fascinating cross-linked architectures are successfully gained in a facile and controllable way via electrospinning polyacrylonitrile (PAN) /dicyandiamide (DICY) composite nanofibers followed by carbonation and a CO₂ activation process. The unique inter-bonded structures and heteroatoms contents could be easily controlled by adjusting the preoxidation temperature applied in the calcining procedure and the addition of DICY. Significantly, the resultant samples display hierarchical pores with micro/meso/macropores, abundant N, O species doped and unique fiber-fiber interconnections, which considerably boost the electrochemical properties. As an electrode material, the activated N-doped cross-linked carbon nanofibers (ANCLCNFs) show a high capacitance of 323 F g^-1 with a current density of 0.5 A g^-1, excellent rate capacity (230.1 F g^-1 at 20 A g^-1) and long-term duration (over 95% after 10000 cycles). Furthermore, the symmetrical supercapacitor delivers a maximum energy density of 14.3 Wh kg^-1 at a power density of 162.5 W kg^-1.

An N-doped porous carbon network with a multidirectional structure as a highly efficient metal-free catalyst for the oxygen reduction reaction

Metal-free catalysts have gained substantial attention as a promising candidate to replace the expensive platinum (Pt) catalysts for the oxygen reduction reaction (ORR) which is a key process in low temperature fuel cells. Development of highly efficient and mass-producible N-doped carbon catalysts, however, remains to be a great challenge. In this study, N-doped porous carbon (NPC) materials were synthesized via a simple, cost-effective and scalable method for mass production by using the d-gluconic acid sodium salt, pyrrole, Triton X-100 and KOH. The resulting NPC possessed a multidirectional porous carbon network (SBET: 1026.6 m2 g-1, Vt: 1.046 cm3 g-1) with hierarchical porosity and plenty of graphitic N species (49.1%). Electrochemical tests showed that the NPC itself was highly active for the ORR under alkaline and acidic conditions via a four electron pathway for the complete reduction of O2 in water. More importantly, this NPC catalyst demonstrated better performance than commercial Pt/C catalysts in terms of long-term durability and methanol tolerance under both conditions.

Preparation and characterization of activated carbon derived from the Borassus flabellifer flower as an electrode material for supercapacitor applications

Activated carbon is prepared by a two stage process with H₃PO₄ activating agent using the precursor material Borassus flabellifer flower as an electrode material for supercapacitors.

Anchovy-derived nitrogen and sulfur co-doped porous carbon materials for high-performance supercapacitors and dye-sensitized solar cells

Anchovy-derived nitrogen and sulfur co-doped porous carbons (AnCs) were prepared by a simple carbonization and alkali activation method for use as superior electrodes in supercapacitors and dye-sensitized solar cells (DSSCs).