Keqin Yang

Porous carbon nanotubes etched by water steam for high-rate large-capacity lithium–sulfur batteries

The current investigation of lithium–sulfur (Li–S) batteries faces three practical problems: (1) the poor conductivity of sulfur; (2) the notorious shuttle mechanism; and (3) the volume variation of the sulfur cathode. In principle, carbon nanotubes (CNTs) have a strong potential for improving sulfur usage because of their high electrical conductivity. Furthermore, opening holes in CNTs or creating pores on the walls is also a useful approach to not only enhance the diffusion of Li ions, but also enable more sulfur to fill the interior of the CNTs, which would be beneficial in retaining the soluble poly-sulfide intermediates and accommodate volume variations. Herein, we designed a mild one-step oxidation approach to create porous CNTs (PCNTs) through a chemical reaction between CNTs and rare oxygen sourced from a nebulized water stream at high temperatures. The higher specific surface area and pore volume values confirmed that PCNTs had significant porosity, compared with raw CNTs. When the PCNTs–S composites were tested as cathode materials in Li–S batteries, the cathode with 78 wt% S content exhibited an initial reversible capacity of 1382 mA h g−1 at 0.2 C. Furthermore, a reversible capacity of 150 mA h g−1 can be preserved, even at a very high current rate of 15 C. More importantly, it is also confirmed that a cathode with 89 wt% S content unexpectedly delivered a reversible capacity as high as 1165 mA h g−1sulfur/830 mA h g−1electrode at the initial cycle, and 792 mA h g−1sulfur/564 mA h g−1electrode after 200 cycles at a current rate of 0.2 C. To the best of our knowledge, such a high rate performance (15 C) and S loading (89 wt%) in cathodes of advanced Li–S batteries have been infrequently reported in previous research.

A nickel hydroxide-coated 3D porous graphene hollow sphere framework as a high performance electrode material for supercapacitors

A three-dimensional (3D) porous graphene hollow sphere (PGHS) framework has been fabricated via a hard template method and used to anchor α-Ni(OH)2 nanoparticles with the size of about 4 nm through electrochemical deposition. It is found that a 3D PGHS framework can improve the capacitive performance of Ni(OH)2 effectively. In hybrid materials, α-Ni(OH)2 achieves the high specific capacitance of 2815 F g(-1) at a scan rate of 5 mV s(-1) and 1950 F g(-1) even at 200 mV s(-1) with a capacitance retention of about 70%, indicating that the α-Ni(OH)2-coated 3D PGHS framework exhibits high rate capability. The excellent performance of such hybrid material is believed to be due to the smaller size of Ni(OH)2 nanoparticles and the PGHS framework with large specific surface area promoting efficient electron transport and facilitating the electrolyte ions migration. These impressive results suggest that the composite is a promising electrode material for an efficient supercapacitor.

3D hierarchical nitrogen-doped carbon nanoflower derived from chitosan for efficient electrocatalytic oxygen reduction and high performance lithium–sulfur batteries

Despite diverse carbon materials being intensively applied in energy storage and conversion, efficient optimization of the carbon structure to further improve its performance is still a great challenge. Herein, we design and fabricate a highly uniform 3D hierarchical N-doped carbon nanoflower (NCNF) using low-cost chitosan as the nitrogen and carbon source by a silica template method. The as-prepared NCNF with abundant meso-porous channels displays a high surface area (907 m2 g−1) and large pore volume (1.85 cm3 g−1), thereby demonstrating high performance as a bifunctional material for the electrocatalytic oxygen reduction reaction (ORR) and in lithium–sulfur batteries. As a metal-free ORR electrocatalyst, the NCNF exhibits excellent electrochemical activity comparable to that of commercial Pt/C (20 wt%), and much better methanol tolerance and durability. As sulfur accommodation for a Li–S battery cathode, the NCNF high loading content of sulfur (80 wt%) achieves an extremely high capacity (1633 mA h g−1 at 0.2C), excellent rate capability (916 mA h g−1 at 5C) and good cycling performance with a capacity decay of 0.07% per cycle over 500 cycles at 1C. Even when the area density is improved to 4.5 mgsulfur cm−2, the battery delivers a high areal capacity of ∼5.5 mA h cm−2 (0.37 mA cm−2) and still maintains ∼3 mA h cm−2 after 200 cycles with a smaller capacity decay of 0.07% per cycle at a high area current density of 3.77 mA cm−2. Significantly, the carbon materials recycled from the Li–S cathode after 500 cycles are reused as ORR electrocatalysts, displaying more excellent electrocatalytic activity than Pt/C (20 wt%).

Controllable synthesis of highly uniform flower-like hierarchical carbon nanospheres and their application in high performance lithium–sulfur batteries

Elemental sulfur cathodes for lithium/sulfur batteries are receiving intense interest owing to their high theoretical capacity and energy density. However, they still suffer from severe capacity fading and moderate rate capability. Herein, we provide rational design and controllable fabrication of highly uniform flower-like hierarchical carbon nanospheres (FCNS) for sulfur accommodation for lithium/sulfur battery cathodes. The as-prepared three dimension FCNS with a size of around 200 nm seem to be assembled by petal-like carbon nanosheets with a thickness of about 4 nm, forming many mesoporous channels, which lead to their high surface area and large pore volume. With such a tailor-made structure, FCNS/sulfur composite cathodes with high sulfur-loading (81 wt%) deliver high specific capacity, long cycling life and excellent rate capability. Particularly, N-doped flower-like carbon nanospheres (NFCNS) with higher surface area (1223 m2 g−1) and larger pore volume (2.33 cm3 g−1) are also fabricated by treating with NH3 and used to host sulfur in lithium–sulfur battery cathodes, exhibiting more excellent rate capability (829 mA h g−1 at 5C) and cycling stability with a decay of 0.03% per cycle over 200 cycles at 1C. Even though the area density is improved to 2.5 mg sulfur per cm2, the battery still has a decay of 0.056% per cycle over 200 cycles.

Direct fabrication of metal-free hollow graphene balls with a self-supporting structure as efficient cathode catalysts of fuel cell

Despite the good progress in developing carbon catalysts for oxygen reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing hollow graphene balls with a self-supporting structure is considered to be an ideal method to inhibit graphene stacking and improve their catalytic performance. Herein, we fabricated metal-free hollow graphene balls with a self-supporting structure, through using a new strategy that involves direct metal-free catalytic growth from assembly of SiO2 spheres. To our knowledge, although much researches involving the synthesis of graphene balls have been reported, investigations into the direct metal-free catalytic growth of hollow graphene balls are rare. Furthermore, the electrocatalytic performance shows that the resulting hollow graphene balls have significantly high catalytic activity. More importantly, such catalysts also possess much improved stability and better methanol tolerance in alkaline media during the ORR compared with commercial Pt/C catalysts. The outstanding performances coupled with an easy and inexpensive preparing method indicated the great potential of the hollow graphene balls with a self-supporting structure in large-scale applications of fuel cell.Graphical AbstractHollow graphene balls with a self-supporting structure have been successfully fabricated, through using a new strategy that involves direct metal-free catalytic growth from 3D assembly of SiO2 spheres. The hollow graphene balls can exhibit a high catalytic activity, long-term stability, and an excellent methanol tolerance for the oxygen reduction reaction

Neuron-Inspired Interpenetrative Network Composed of Cobalt–Phosphorus-Derived Nanoparticles Embedded within Porous Carbon Nanotubes for Efficient Hydrogen Production

The ongoing search for cheap and efficient hydrogen evolution reaction (HER) electrocatalysts to replace currently used catalysts based on Pt or its alloys has been considered as an prevalent strategy to produce renewable and clean hydrogen energy. Herein, inspired by the neuron structure in biological systems, we demonstrate a novel fabrication strategy via a simple two-step method for the synthesis of a neuronlike interpenetrative nanocomposite network of Co-P embedded in porous carbon nanotubes (NIN-Co-P/PCNTs). It is found that the interpenetrative network provides a natural transport path to accelerate the hydrogen production process. The embedded-type structure improves the utilization ratio of Co-P and the hollow, tubelike, and porous structure of PCNTs further promote charge and reactant transport. These factors allow the as-prepared NIN-Co-P/PCNTs to achieve a onset potential low to 43 mV, a Tafel slope as small as 40 mV/decade, an excellent stability, and a high turnover frequency value of 3.2 s(-1) at η = 0.2 V in acidic conditions. These encouraging properties derived from the neuronlike interpenetrative network structure might offer new inspiration for the preparation of more nanocomposites for applications in other catalytic and optoelectronic field.