Inner porous carbon nanofibers as binder-free electrodes for high-rate supercapacitors

High-rate transition metal-based cathode materials for battery-supercapacitor hybrid devices

With the rapid development of portable electronic devices, electric vehicles and large-scale grid energy storage devices, there is a need to enhance the specific energy density and specific power density of related electrochemical devices to meet the fast-growing requirements of energy storage. Battery-supercapacitor hybrid devices (BSHDs), combining the high-energy-density feature of batteries and the high-power-density properties of supercapacitors, have attracted mass attention in terms of energy storage. However, the electrochemical performances of cathode materials for BSHDs are severely limited by poor electrical conductivity and ion transport kinetics. As the rich redox reactions induced by transition metal compounds are able to offer high specific capacity, they are an ideal choice of cathode materials. Therefore, this paper reviews the currently advanced progress of transition metal compound-based cathodes with high-rate performance in BSHDs. We discuss some efficient strategies of enhancing the rate performance of transition metal compounds, including developing intrinsic electrode materials with high conductivity and fast ion transport; modifying materials, such as inserting defects and doping; building composite structures and 3D nano-array structures; interfacial engineering and catalytic effects. Finally, some suggestions are proposed for the potential development of cathodes for BSHDs, which may provide a reference for significant progress in the future.

Nanohollow Carbon for Rechargeable Batteries: Ongoing Progresses and Challenges

Among the various morphologies of carbon-based materials, hollow carbon nanostructures are of particular interest for energy storage. They have been widely investigated as electrode materials in different types of rechargeable batteries, owing to their high surface areas in association with the high surface-to-volume ratios, controllable pores and pore size distribution, high electrical conductivity, and excellent chemical and mechanical stability, which are beneficial for providing active sites, accelerating electrons/ions transfer, interacting with electrolytes, and giving rise to high specific capacity, rate capability, cycling ability, and overall electrochemical performance. In this overview, we look into the ongoing progresses that are being made with the nanohollow carbon materials, including nanospheres, nanopolyhedrons, and nanofibers, in relation to their applications in the main types of rechargeable batteries. The design and synthesis strategies for them and their electrochemical performance in rechargeable batteries, including lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and lithium-sulfur batteries are comprehensively reviewed and discussed, together with the challenges being faced and perspectives for them.

Nickel nanoparticles embedded in porous carbon nanofibers and its electrochemical properties

Flexible porous carbon nanofibers containing nickel nanoparticles were synthesized by direct carbonization of electrospun Ni-MOFs/polyacrylonitrile fibers. The as-synthesized composite nanofibers were employed as binder-free electrodes, and exhibit high specific capacitance (up 672 F g^-1 at current density of 2 A g^-1) and superior rate capability (57% capacitance retention from current density of 2-10 A g^-1), which may be attributed to their binder-free nature, unique one-dimensional (1D) structure and highly dispersed electrochemically active nickel nanoparticles. Furthermore, a symmetric supercapacitor was assembled using the fiber electrodes in 6 M KOH, and the energy density of 17.8 Wh kg^-1 was achieved in a potential window of 1.5 V. This self-standing fiber with abundant mesopores and macropores is expected to become a promising electrode material for high-performance supercapacitors.

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.

Paper-Derived Flexible 3D Interconnected Carbon Microfiber Networks with Controllable Pore Sizes for Supercapacitors

Heteroatom-doped three-dimensional (3D) carbon fiber networks have attracted immense interest because of their extensive applications in energy-storage devices. However, their practical production and usage remain a great challenge because of the costly and complex synthetic procedures. In this work, flexible B, N, and O heteroatom-doped 3D interconnected carbon microfiber networks (BNOCs) with controllable pore sizes and elemental contents were successfully synthesized via a facile one-step "chemical vapor etching and doping" method using cellulose-made paper, the most abundant and cost-effective biomass, as an original network-frame precursor. Under a rational design, the BNOCs exhibited interconnected microfiber-network structure as expressways for electron transport, spacious accessible surface area for charge accumulation, abundant mesopores and macropores for rapid inner-pore ion diffusion, and lots of functional groups for additional pseudocapacitance. Being applied as binder-free electrodes for supercapacitors, BNOC-based supercapacitors not only revealed a high specific capacitance of 357 F g^-1, a high capacitance retention of 150 F g^-1 at 200 A g^-1, a high energy density of 12.4 W h kg^-1, and a maximum power density of 300.6 kW kg^-1 with an aqueous electrolyte in two-electrode configuration but also exhibited a high specific capacitance of up to 242.4 F g^-1 in an all-solid-state supercapacitor.