Jiqi Zheng

Hydrothermal synthesis of vanadium dioxides/carbon composites and their transformation to surface-uneven V2O5 nanoparticles with high electrochemical properties

Vanadium dioxides/carbon composites composed of vanadium dioxides@carbon core–shell structures and amorphous carbon spheres were successfully synthesized using glucose as the carbon sources by a facile one-step hydrothermal route. Then vanadium dioxides/carbon composites were converted to surface-uneven V2O5 nanoparticles by the calcination in air atmospheres. The amorphous carbon reacting with O2 in the air to release gas results in remaining V2O5 nanoparticles possessing broken, rough and poral structures. The electrochemical properties of surface-uneven V2O5 nanoparticles as supercapacitor electrodes were measured by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) both in the aqueous and organic electrolyte. Surface-uneven V2O5 nanoparticles exhibit the specific capacitance of 406 F g−1 at the current density of 0.2 A g−1 and retain 246 F g−1 even at high current density of 10 A g−1. The influence of the calcined temperature and time on the specific capacitance, phase and morphology of the products were discussed in detail. The results revealed that the calcination at 400 °C for 4 h with comparatively low ratio of V5+/V4+ are favorable for surface-uneven V2O5 nanoparticles with the high electrochemical property. During the cycle performance, the specific capacitances of V2O5 nanoparticles after 100 cycles are 13.8% and 98.5% of the initial discharge capacity in the aqueous and organic electrolyte, respectively, indicating the cycle performance is significantly improved in organic electrolyte. It turns out that surface-uneven V2O5 nanoparticles are an ideal material for supercapacitor electrode in the present work.

3D hierarchical porous V3O7·H2O nanobelts/CNT/reduced graphene oxide integrated composite with synergistic effect for supercapacitors with high capacitance and long cycling life

V3O7·H2O possesses the merit of high specific capacitance, but weakness of cycling stability and low conductivity inhibit its application for energy devices, which requires the addition of carbon materials like carbon nanotubes (CNT) or graphene with properties of high conductivity and brilliant cycling stability to obtain high-performance composites. Since CNT or GO-based binary V3O7·H2O materials have been rarely studied with limited specific capacitance, we developed a novel, highly porous V3O7·H2O nanobelts/CNT/reduced graphene oxide (V3O7·H2O/CNT/rGO) ternary composite with a three-dimensional (3D) hierarchical micro-structure by a single step, facile hydrothermal process and self-assembly method with outstanding electrochemical performances. During the hydrothermal process, CNT-anchored V3O7·H2O nanobelts have been incorporated on the surface of rGO through in situ growth with preferred orientation, forming a 3D hierarchical porous structure composed mostly of mesopores and exhibiting enlarged specific surface area up to 53.7 m2·g-1. The well-designed V3O7·H2O nanobelts also display excellent adhesion with CNT/rGO, which leads to reduced resistance resulted from the synergistic effect of pseudocapacitors (V3O7·H2O nanobelts) and electric double-layer capacitors (EDLCs) (CNT/rGO) and large specific area with sufficient active sites ensure the composite to brilliant capacitive behavior. Applied to SCs, the ternary composite exhibits outstanding electrochemical performance with higher specific capacitance (685 F·g-1 at 0.5 A·g-1), higher energy density (34.3 W·h·kg-1) and extremely prominent cycle stability (99.7% of initial specific capacitance after 10,000 cycles) compared to those of most similar binary materials. Results suggest that the V3O7·H2O/CNT/rGO ternary composite is a promising candidate for electrode materials applying to high-performance supercapacitors.