Hui Chai

The glucose-assisted synthesis of a graphene nanosheet–NiO composite for high-performance supercapacitors

A green and facile approach was used for the synthesis of a graphene nanosheet NiO composite. Using glucose as the green reduction agent, graphene oxide was reduced to graphene and formed a membrane-like NiO–GNS–GC composite via a hydrothermal method. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, the electrochemical performance of the composite was evaluated by cyclic voltammetry and galvanostatic charge–discharge. The obtained NiO–GNS–GC composite exhibits the highest specific supercapacitance of 1495 F g−1 at a current density of 0.5 A g−1 in 6 M KOH, which is much higher than that of pure NiO (478 F g−1) and pure GNS (92 F g−1) along with 90% specific capacitance remaining after 1000 cycles. This could be attributed to the layered sandwich nanostructure and the fact that during the hydrothermal process glucose transformed to a kind of amorphous carbon which, as a spacer, could stabilize the structure of the as-prepared GNS.

Effective microwave-assisted synthesis of graphene nanosheets/NiO composite for high-performance supercapacitors

A facile and fast strategy was used for synthesis of a petal-like graphene nanosheets (GNS)/NiO composite. Using this strategy, graphene oxide was reduced to graphene and formed the GNS/NiO composite via a microwave-assisted method without a complicated procedure and any controlling-agent. The product was characterized by X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analysis results have confirmed that petal-like NiO sheets are well dispersed on the surfaces of graphene nanosheets. The as-prepared composite was electrochemically tested by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy as an electrode material for supercapacitors. The GNS/NiO composite exhibits a specific supercapacitance of 799 F g−1 at a current density of 0.3 A g−1 in 6 M KOH electrolyte and a long cycle life, along with 90% specific capacitance remaining after 1000 cycles. The electrochemical performance of the composite was significantly improved compared to bare graphene and NiO. This could be attributed to their architecture. The results presented here suggest that the GNS/NiO composite could have potential applications in high energy storage systems.

Design and synthesis of carbonized polypyrrole-coated graphene aerogel acting as an efficient metal-free catalyst for oxygen reduction

Carbonized polypyrrole (PPy)-coated graphene aerogel (CPGA) was synthesized using graphene oxide and fresh pyrrole (Py) as raw materials in a hydrothermal process without any oxidant. After annealing at a high temperature, the as-prepared aerogel had a porous 3D network, contained a high percentage of nitrogen, and delivered a promising electrochemical catalytic activity. Most importantly, the annealing temperature had a significant impact on the electrochemical performance of the obtained materials. Among these products, CPGA-600 displayed the most positive onset potential and the highest peak current density for an oxygen reduction reaction (ORR). The electrocatalytic mechanism of the as-prepared materials for ORR in an alkaline electrolyte was also analyzed in detail. The results indicated that pyrrolic-N and highly graphitized carbon structures are mainly responsible for the enhanced ORR activity of metal-free N-doped carbon materials.

Facile synthesis of Mn3O4-rGO hybrid materials for the high-performance electrocatalytic reduction of oxygen

In this study, a highly active oxygen reduction reaction (ORR) electrocatalyst (MG-15) of Mn3O4 nanoparticles (NPs) supported by reduced graphene oxide (rGO) has been fabricated through one-step microwave-assisted synthetic route. Upon microwave-assisted synthesis, the formation of Mn3O4 NPs and the reduction of GO occurs simultaneously. Transmission electron microscope (TEM) profile reveals that Mn3O4 NPs are uniformly distributed evenly on the winkled rGO sheets. The MG-15 hybrid materials show enhanced electrocatalytic activity compared to rGO, Mn3O4 NPs and the mixture of rGO and Mn3O4 NPs, which indicates that the synergistic effect of Mn3O4 and rGO enhances the overall performance. Additional, the oxygen reduction peak of the MG-15 catalyst in a 0.1M KOH solution is tested at -0.15V, which is more positive than Pt/C (-0.154V). The onset potential of the MG-15 is close to Pt/C. Most importantly, the mechanism analysis shows that it favors the 4e- pathway for ORR. Furthermore, the MG-15 also exhibit excellent durability and methanol.

High-performance supercapacitors based on conductive graphene combined with Ni(OH)2 nanoflakes

A green and facile strategy is reported for the synthesis of a three-dimensional (3D) graphene nanosheets (GNS)/Ni(OH)2 composite for use as a supercapacitor material. During this process, graphene oxide was reduced to graphene and Ni(OH)2 was attached in it to form the GNS/Ni(OH)2 composite via a chemical precipitation route without any complicated procedures. The product was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The analyses indicated that the Ni(OH)2 sheets were well interwoven on the surfaces of the graphene nanosheets. Furthermore, the composite was electrochemically tested by cyclic voltammetry, galvanostatic charge/discharge, specific capacitance, and by assessing its cycle life. The GNS/Ni(OH)2 composite exhibited a high specific supercapacitance of 2053 F g−1 at a current density of 0.3 A g−1 in 6 M KOH electrolyte and a long cycle life, along with 97% specific capacitance remaining after 1000 cycles. The GNS/Ni(OH)2 composite had superb electrochemical performance compared to bare Ni(OH)2, which could be attributed to its architecture. These results suggest that the GNS/Ni(OH)2 composite could have potential application as a supercapacitor material.

3D interpenetrating macroporous graphene aerogels with MnO2 coating for supercapacitors

We report a novel interpenetrating three-dimentional (3D) macroporous graphene aerogels (GAs) composite consisting of MnO2 wrapping for potential supercapacitor applications. In this article, the macroporous GAs holds abundant of interconnected macroporous and has excellent intrinsic high electrical conductivity resulting from the graphene. Therefore, employing the GAs as the substrates to combine with MnO2 will exhibit excellent electronic conductivity and effective ion-transport capability. The 3D macroporous MnO2/GAs composite has a specific capacitance of 200 F g-1. Thus, the corresponding 3D porous hierarchical GAs/MnO2 materials have a great potential to be used in the field of supercapacitors. This strategy could also be used for other materials to improve the performance of supercapacitors.

Sulfonated graphene oxide as an adsorbent for removal of Pb2+ and methylene blue

One of the major challenges encountered in some conventional nano-structured adsorbents such as graphene oxide (GO) and graphene is the structural limits including serious aggregation and hydrophilic surface in water. And the sulfanilic acid functionalized graphene oxide (SGO) can powerfully attract positively charged pollutants. Therefore, the SGO served as an adsorbent to remove dyes and toxic metal ions from aqueous solution were intensively investigated. At the same time the reduced sulfonated graphene oxide (rSGO), reduced graphene oxide (rGO), and graphene oxide (GO) were performed as the comparative samples. The results showed that the maximum adsorption capacities of SGO were 2530 mg/g for methylene blue and 415 mg/g for Pb2+, which was much higher than that of the contrast samples and adsorbents reported in literatures. The adsorption of SGO was investigated systematically including the saturated adsorption capacities, isotherm, and kinetic adsorption process. The SGO displayed high adsorption efficiency and superior adsorption capacity toward metal ions and dyes, which is mainly attribute to the good dispersibility and the multiple adsorption sites of SGO. These results are promising not only providing effective adsorbing heavy metal ions and organic dyes, but also gaining insights into adsorption mechanism of graphene materials.

An effective bifunctional electrocatalysts: Controlled growth of CoFe alloy nanoparticles supported on N-doped carbon nanotubes

Exploring efficient and inexpensive bi-functional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a critical work for developing the fuel cells and the metal-air batteries. Nitrogen-doped carbon and transition metal (Fe or Co) has been demonstrated as promising catalyst due to the synergetic effect. In this work, the CoFe alloy nanoparticles supported on N-doped carbon nanotubes (CoFe@NCNTs) are synthetized by one-step annealing the precursors, without any templates. The as-prepared materials show both extraordinary electrocatalysis activity for ORR and OER in alkaline solution: a diffusion current density of -5.53 mA cm-2, approximate four-electron selectivity as ORR catalyst, a potential of 0.842 V at 10 mA cm-2. Specifically, the CoFe@NCNTs present a Tafel slope of 60.16 mV dec-1 as OER catalyst and the variance (ΔE) is below 1.017 V in 0.1 M KOH for the OER and ORR.