Quan-Hong Yang

Low-Temperature Exfoliated Graphenes: Vacuum-Promoted Exfoliation and Electrochemical Energy Storage

A preheated high-temperature environment is believed to be critical for a chemical-exfoliation-based production of graphenes starting from graphite oxide, a belief that is based on not only experimental but also theoretical viewpoints. A novel exfoliation approach is reported in this study, and the exfoliation process is realized at a very low temperature, which is far below the proposed critical exfoliation temperature, by introducing a high vacuum to the exfoliation process. Owing to unique surface chemistry, low-temperature exfoliated graphenes demonstrate an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the high-temperature exfoliated ones. The low-temperature exfoliation approach presents us with a possibility for a mass production of graphenes at low cost and great potentials in energy storage applications of graphene-based materials.

Hydrogen storage in carbon nanotubes

Abstract Hydrogen is the cleanest, sustainable and renewable energy carrier, and a hydrogen energy system is expected to progressively replace the existing fossil fuels in the future, the latter are being depleted very fast and causes severe environmental problems. In particular, one potential use of hydrogen lies in powering zero-emission vehicles via a proton exchange membrane fuel cell to reduce atmosphere pollution. To achieve this goal feasible onboard hydrogen storage systems have to be developed. The recent discovery of the high and reversible hydrogen storage capacity of carbon nanotubes makes such a system very promising. In this overview, theoretical predictions and experimental results on the hydrogen uptake of carbon nanotubes and nanofibers are summarized, and we point out that, in order to accelerate the development of carbon nanotubes and nanofibers as a practical hydrogen storage medium in fuel cell-driven vehicles, many efforts have to be made to reproduce and verify the results both theoretically and experimentally, and to investigate their volumetric capacity, cycling characteristics and release behavior.

On the origin of the stability of graphene oxide membranes in water

Graphene oxide (GO) films are known to be highly stable in water and this property has made their use in membrane applications in solution possible. However, this state of affairs is somewhat counterintuitive because GO sheets become negatively charged on hydration and the membrane should disintegrate owing to electrostatic repulsion. We have now discovered a long-overlooked reason behind this apparent contradiction. Our findings show that neat GO membranes do, indeed, readily disintegrate in water, but the films become stable if they are crosslinked by multivalent cationic metal contaminants. Such metal contaminants can be introduced unintentionally during the synthesis and processing of GO, most notably on filtration with anodized aluminium oxide filter discs that corrode to release significant amounts of aluminium ions. This finding has wide implications in interpreting the processing-structure-property relationships of GO and other lamellar membranes. We also discuss strategies to avoid and mitigate metal contamination and demonstrate that this effect can be exploited to synthesize new membrane materials.

Adsorption of lead(II) ions from aqueous solution on low-temperature exfoliated graphene nanosheets.

Graphene nanosheets (GNSs) that were obtained by vacuum-promoted low-temperature exfoliation were used to adsorb lead ions from an aqueous system. The pristine and thermally modified GNSs were characterized with scanning electron microscopy observation and X-ray photoelectron spectroscopy analysis. It was interestingly found that the adsorption against lead ions was enhanced by heat treatment, although the oxygen complexes of GNSs showed a significant decrease. In addition, lead ion uptake resulted in an increase in the pH value of the solution. It is supposed that the Lewis basicity of GNSs is improved by heat treatment under a high vacuum, in favor of simultaneous adsorption of lead ions and protons onto GNSs.

Multi-step purification of carbon nanotubes

An efficient purification procedure for multi-walled carbon nanotubes (MWNTs) synthesized by the floating catalyst method was discussed. The process involves ultra-sonication, heat treatment in hot water, bromination, oxidation and acid treatment. Most of amorphous carbon, multishell carbon nanocapsules as well as metal particles were successfully removed from the MWNT product. With this procedure, MWNTs with purity of more than 94% were obtained and the yield could approach 50%. It was found that bromination took an important role in the purification of MWNTs. Transmission electron microscopy, XPS and thermo-gravimetric analysis were used to evaluate the purification process of MWNTs. The mechanism of bromination on purification of the MWNTs was also discussed

Flexible and planar graphene conductive additives for lithium-ion batteries

Graphene is introduced into a lithium-ion battery (LIB) as a type of novel but powerful planar conductive additive and the flexible graphene-based conducting network is characterized by a novel “plane-to-point” conducting mode with exceptional electron transport properties and unique geometrical nature (a soft and ultrathin planar structure). With a much lower fraction of graphene additives than those of commercial carbon based additives, the graphene-introduced LiFePO4 cathode shows better charge/discharge performance than commercial cases. Graphene also shows a better performance compared to carbon nanotubes, another type of novel conductive additive with similar fractions. These results present us an indication that graphene will possibly find early application as a flexible and planar conductive additive in high performance LIBs, as our further efforts have shown that a graphene-introduced battery is of better performance as compared to real commercial batteries with conventional additives.

Macroscopic 3D Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution

A macroscopic 3D porous graphitic carbon nitride (g-CN) monolith is prepared by the one-step thermal polymerization of urea inside the framework of a commercial melamine sponge and exhibits improved photocatalytic water-splitting performance for hydrogen evolution compared to g-CN powder due to the 3D porous interconnected network, larger specific surface area, better visible light capture, and superior charge-separation efficiency.

Facile synthesis of Li4Ti5O12/C composite with super rate performance

The Li4Ti5O12/C composite with lump morphology and excellent rate performance are synthesized using a facile hydrothermal method followed by a low temperature heat treatment. In the hydrothermal process, the introduction of cetyltrimethylammonium bromide (CTAB) as a surfactant significantly improves the rate performance of Li4Ti5O12/C composite as anode material for lithium ion battery (LIB). The specific capacities of the obtained composite at charge and discharge rates of 0.1, 1, 5, 10 and 20 C are 176, 163, 156, 151 and 136 mA h g−1, respectively, which is apparently larger than those of the Li4Ti5O12/C free from CTAB in the preparation. The Li4Ti5O12/C prepared in presence of CTAB also shows excellent cycling performance at high rate, which is attributed to its larger diffusion coefficient of lithium ion (6.82 × 10−12 cm2 s−1) and smaller charge-transfer resistance (Rct) (19.2 Ω) than those of the composite (1.22 × 10−13 cm2 s−1 and 50.2 Ω) free from CTAB in the preparation. The Li4Ti5O12 particles obtained in presence of CTAB are coated uniformly by a thin carbon layer with a thickness of ∼1 nm, whereas the Li4Ti5O12 particles obtained in absence of CTAB are covered by relatively thick surface layers with a thickness of ∼2.5 nm, which is too thick, blocks the lithium ion diffusion and leads to low ionic conductivity.

Gassing in Li 4 Ti 5 O 12 -based batteries and its remedy

Destructive gas generation with associated swelling has been a major challenge to the large-scale application of lithium ion batteries (LIBs) made from Li4Ti5O12 (LTO) anodes. Here we report root causes of the gassing behavior, and suggest remedy to suppress it. The generated gases mainly contain H2, CO2 and CO, which originate from interfacial reactions between LTO and surrounding alkyl carbonate solvents. The reactions occur at the very thin outermost surface of LTO (111) plane, which result in transformation from (111) to (222) plane and formation of (101) plane of anatase TiO2. A nanoscale carbon coating along with a stable solid electrolyte interface (SEI) film around LTO is seen most effective as a barrier layer in suppressing the interfacial reaction and resulting gassing from the LTO surface. Such an ability to tune the interface nanostructure of electrodes has practical implications in the design of next-generation high power LIBs.

Towards superior volumetric performance: design and preparation of novel carbon materials for energy storage

The volumetric performance of electrochemical energy storage (EES) devices, other than gravimetric performance, is attracting increasing attention due to the fast development of electric vehicles and smart devices. Carbon-based electrodes have advanced the fast development of EES devices while being limited by their low volumetric performance because of their porous structure and the resulting low density. This paper aims to clarify the importance of the volumetric performance and review the most recent progress in advanced EES devices with a high volumetric performance. Strategies for improving the volumetric performance, particularly with carbon-based materials, are also proposed here. The transformation of normal low-density carbons to high-density ones through the assembly of different building blocks is highlighted as a promising remedy for this issue, and their applications in next-generation EES devices (Li–S, Li–air, Na-ion, etc.) are also discussed.