Blocking Probability Based Hierarchical Storage Design in VoD Systems

In this paper, we discuss the relationship between the blocking probabilityand the configuration of storage hierarchy in a Video on Demand ( VoD) server using the queueingmodel we propose. With this model, the optimal configuration of the storage system can be obtained .

A facile one-step approach to hierarchically assembled core-shell-like MnO2@MnO2 nanoarchitectures on carbon fibers： An efficient and flexible electrode material to enhance energy storage

Hierarchical core-shell-like MnO2 nanostructures （NSs） were used to anchor MnO2 hexagonal nanoplate arrays （HNPAs） on carbon cloth （CC） fibers. The NSs were prepared by a novel one-step electrochemical deposition method. Under an external cathodic voltage of -2.0 V for 30 min, hierarchical core-shell-like MnO2-NS-decorated MnO2 HNPAs （MnO2 NSs@MnO2 HNPAs） were uniformly grown on CC with reliable adhesion. The phase purity and morphological properties of the samples were characterized by various physicochemical techniques. At a constant external cathodic voltage, growth of MnO2 NSs@MnO2 HNPAs on CC was carried for different time periods. When utilized as a flexible, robust, and binder-free electrode for pseudocapacitors, the hierarchical core-shell-like MnO2 NSs@MnO2 HNPAs on CC showed clearly enhanced electrochemical properties in 1 M Na2SO4 electrolyte solution. The results indicate that the MnO2 NSs@MnO2 HNPAs on CC have a maximum specific capacitance of 244.54 F/g at a current density of 0.5 A/g with excellent cycling stability compared to that of bare MnO2 HNPAs on CC （112.1 F/g at 0.5 A/g current density）. We believe that the superior charge storage performance of the pseudocapacitive electrode can be mainly attributed to the hierarchical MnO2 NSs@MnO2 HNPAs building blocks that have a large specific surface area, offering additional electroactive sites for efficient electrochemical reactions. The facile and single-step approach to growth of hierarchical pseudocapacitive materials on textile based electrodes opens up the possibility for the fabrication of high-performance flexible energy storage devices.

Preparation and Li^+ storage properties of hierarchical hollow porous carbon spheres

Hollow ordered porous carbon spheres （HOPCS） with a hierarchical structure were prepared by templating with hollow ordered mesoporous silica spheres （HOMSS）. Scanning electron microscopy （SEM） and transmission electron microscopy （TEM） showed that HOPCS exhibited a spherical hollow morphology. High-resolution TEM, small angle X-ray diffraction （SAXRD） and N2 sorption measurements confirmed that HOPCS inversely replicated the unconnected hexagonal-stacked pore structure of HOMSS, and possessed ordered porosity. HOPCS exhibited a higher storage capacity for Li^＋ ion battery （LIB） of 527.6 mA h/g, and good cycling performance. A large capacity loss during the first discharge-charge cycle was found attributed to the high content of micropores. The cycling performance was derived from the hierarchical structure.

Construction of hierarchical three-dimensional interspersed flower-like nickel hydroxide for asymmetric supercapacitors

Low-cost and easily obtainable electrode materials are crucial for the application of supercapacitors.Nickel hydroxides have recently attracted intensive attention owning to their high theoretical specific capacitance,high redox activity,low cost,and eco-friendliness.In this study,novel three-dimensional （3D） interspersed flower-like nickel hydroxide was assembled under mild conditions.When ammonia was used as the precipitant and inhibitor and CTAB was used as an exfoliation agent,the obtained exfoliated ultrathin Ni（OH）2 nanosheets were assembled into 3D interspersed flower-like nickel hydroxide.In this novel 3D structure,the ultrathin Ni（OH）2 nanosheets not only provided a large contact area with the electrolyte,reducing the polarization of the electrochemical reaction and providing more active sites,but also reduced the concentration polarization in the electrode solution interface.Consequently,the utilization efficiency of the active material was improved,yielding a high capacitance.The electrochemical performance was improved via promoting the electrical conductivity by mixing the as-synthesized Ni（OH）2 with carbon tubes （N-4-CNT electrode）,yielding excellent specific capacitances of 2,225.1 F＆#183;g-1 at 0.5 A＆#183;g-1 in a three-electrode system and 722.0 F＆#183;g-1 at 0.2 A＆#183;g-1 in a two-electrode system.The N-4-CNT//active carbon （AC） device exhibited long-term cycling performance （capacitance-retention ratio of 111.4% after 10,000 cycles at 5 A＆#183;g-1） and a high specific capacitance of 180.5 F＆#183;g-1 with a high energy density of 33.5 W＆#183;h＆#183;kg-1 and a power density of 2,251.6 W＆#183;kg-1.

Facile synthesis of nitrogen-doped graphene aerogels functionalized with chitosan for supercapacitors with excellent electrochemical performance

Three-dimensional porous nitrogen-doped graphene aerogels（NGAs） were synthesized by using graphene oxide（GO） and chitosan via a self-assembly process by a rapid method.The morphology and structure of the as-prepared aerogels were characterized.The results showed that NGAs possesed the hierarchical pores with the wide size distribution ranging from mesopores to macropores.The NGAs carbonized at different temperature all showed excellent electrochemical performance in 6 mol/L KOH electrolyte and the electrochemical performance of the NGA-900 was the best.When working as a supercapacitor electrode,NGA-900 exhibited a high specific capacitance（244.4 F/g at a current density of 0.2 A/g）,superior rate capability（51.0% capacity retention） and excellent cycling life（96.2% capacitance retained after 5000 cycles）.