Ultra-High Energy Density Hybrid Supercapacitors Using MnO2/Reduced Graphene Oxide Hybrid Nanoscrolls.

Manganese oxide (MnO2) is a promising material for supercapacitor applications, with a theoretical ultra-high energy density of 308 Wh/kg. However, such ultra-high energy density has not been achieved experimentally in MnO2-based supercapacitors because of several practical issues, such as low electrical conductivity of MnO2, incomplete utilization of MnO2, and dissolution of MnO2. The present study investigates the potential of MnO2/reduced graphene oxide (rGO) hybrid nanoscroll (GMS) structures as electrode material for overcoming the difficulties and for developing ultra-high-energy storage systems. A hybrid supercapacitor, comprising MnO2/rGO nanoscrolls as anode material and activated carbon (AC) as a cathode, is fabricated. The GMS/AC hybrid supercapacitor exhibited enhanced energy density, superior rate performance, and promising Li storage capability that bridged the energy-density gap between conventional Li-ion batteries (LIBs) and supercapacitors. The fabricated GMS/AC hybrid supercapacitor demonstrates an ultra-high lithium discharge capacity of 2040 mAh/g. The GMS/AC cell delivered a maximum energy density of 105.3 Wh/kg and a corresponding power density of 308.1 W/kg. It also delivered an energy density of 42.77 Wh/kg at a power density as high as 30,800 W/kg. Our GMS/AC cell's energy density values are very high compared with those of other reported values of graphene-based hybrid structures. The GMS structures offer significant potential as an electrode material for energy-storage systems and can also enhance the performance of the other electrode materials for LIBs and hybrid supercapacitors.

An Ultra-High-Energy Density Supercapacitor; Fabrication Based on Thiol-functionalized Graphene Oxide Scrolls.

Present state-of-the-art graphene-based electrodes for supercapacitors remain far from commercial requirements in terms of high energy density. The realization of high energy supercapacitor electrodes remains challenging, because graphene-based electrode materials are synthesized by the chemical modification of graphene. The modified graphene electrodes have lower electrical conductivity than ideal graphene, and limited electrochemically active surface areas due to restacking, which hinders the access of electrolyte ions, resulting in a low energy density. In order to solve the issue of restacking and low electrical conductivity, we introduce thiol-functionalized, nitrogen-doped, reduced graphene oxide scrolls as the electrode materials for an electric double-layer supercapacitor. The fabricated supercapacitor exhibits a very high energy/power density of 206 Wh/kg (59.74 Wh/L)/496 W/kg at a current density of 0.25 A/g, and a high power/energy density of 32 kW/kg (9.8 kW/L)/9.58 Wh/kg at a current density of 50 A/g; it also operates in a voltage range of 0~4 V with excellent cyclic stability of more than 20,000 cycles. By suitably combining the scroll-based electrode and electrolyte material, this study presents a strategy for electrode design for next-generation energy storage devices with high energy density without compromising the power density.

Self-rectifying bipolar resistive switching memory based on an iron oxide and graphene oxide hybrid.

A resistive random access memory (RRAM) device with self-rectifying I-V characteristics was fabricated by inserting a silicon nitride (Si3N4) layer between the bottom electrode and solution-processed active material of an iron oxide-graphene oxide (FeOx-GO) hybrid. The fabricated Au/Ni/FeOx-GO/Si3N4/n+-Si memory device exhibited an excellent resistive switching ratio and a rectification ratio higher than 104. In the Au/Ni/FeOx-GO/Si3N4/n+-Si device, resistive switching occurs in both the FeOx-GO and Si3N4 layers separately, resulting in a highly uniform and stable switching performance. The resistive switching from a high resistance state to a low resistance state in the Au/Ni/FeOx-GO/Si3N4/n+-Si device occurs through a trap-assisted tunneling process in the Si3N4 layer, enabled by the FeOx-GO layer which prevents diffusion of the migrating Ni metal into the switching nitride layer. The intrinsic self-rectifying characteristics of our memory devices arise from the asymmetric barriers for electrons tunneling into the traps of the Si3N4 layer which is sandwiched between the top and bottom electrodes having dissimilar work functions. Our study confirmed that integrating a suitable dielectric layer into the conventional RRAM cell is an innovative strategy to simplify the architecture and fabrication process to realize self-rectifying crossbar arrays.

High Volumetric Energy Density Hybrid Supercapacitors Based on Reduced Graphene Oxide Scrolls.

The low volumetric energy density of reduced graphene oxide (rGO)-based electrodes limits its application in commercial electrochemical energy storage devices that require high-performance energy storage capacities in small volumes. The volumetric energy density of rGO-based electrode materials is very low due to their low packing density. A supercapacitor with enhanced packing density and high volumetric energy density is fabricated using doped rGO scrolls (GFNSs) as the electrode material. The restacking of rGO sheets is successfully controlled through synthesizing the doped scroll structures while increasing the packing density. The fabricated cell exhibits an ultrahigh volumetric energy density of 49.66 Wh/L with excellent cycling stability (>10 000 cycles). This unique design strategy for the electrode material has significant potential for the future supercapacitors with high volumetric energy densities.

Reduced graphene oxide enwrapped phosphors for long-term thermally stable phosphor converted white light emitting diodes.

The long-term instability of the presently available best commercial phosphor-converted light-emitting diodes (pcLEDs) is the most serious obstacle for the realization of low-cost and energy-saving lighting applications. Emission from pcLEDs starts to degrade after approximately 200 h of operation because of thermal degradation of the phosphors. We propose a new strategy to overcome this thermal degradation problem of phosphors by wrapping the phosphor particles with reduced graphene oxide (rGO). Through the rGO wrapping, we have succeeded in controlling the thermal degradation of phosphors and improving the stability of fabricated pcLEDs. We have fabricated pcLEDs with long-term stability that maintain nearly 98% of their initial luminescence emission intensity even after 800 h of continuous operation at 85 °C and 85% relative humidity. The pcLEDs fabricated using SrBaSi2O2N2:Eu2+ phosphor particles wrapped with reduced graphene oxide are thermally stable because of enhanced heat dissipation that prevents the ionization of Eu2+ to Eu3+. We believe that this technique can be applied to other rare-earth doped phosphors for the realization of highly efficient and stable white LEDs.

Graphene oxide-phosphor hybrid nanoscrolls with high luminescent quantum yield: synthesis, structural, and X-ray absorption studies.

Highly luminescent graphene oxide (GO)-phosphor hybrid thin films with a maximum quantum yield of 9.6% were synthesized via a simple chemical method. An intense luminescence emission peak at 537 nm and a broad emission peak at 400 nm were observed from the GO-phosphor hybrid films. The maximum quantum yield of the emissions from the hybrid films was found to be 9.6%, which is 48 times higher than that of pristine GO films. The GO-phosphor hybrids were prepared via spin-coating and subsequent postannealing of the films, resulting in scrolling of the GO sheets. The resulting GO nanoscrolls exhibited a length of ∼2 µm with nanoscale interior cavities. Transmission electron microscopy and selected-area electron diffraction analyses revealed that the lattice structure of the tubular scrolls is similar to that of carbon nanotubes. While pristine GO films are p-type, in the GO-phosphor hybrids, the Fermi level shifted upward and fell between the HOMO-LUMO gap due to phosphor attachment via C-N bonding. The highly luminescent GO-phosphor hybrids will find important applications in graphene-based optoelectronic devices.

Raman Spectra of Luminescent Graphene Oxide (GO)-Phosphor Hybrid Nanoscrolls.

Graphene oxide (GO)-phosphor hybrid nanoscrolls were synthesized using a simple chemical method. The GO-phosphor ratio was varied to find the optimum ratio for enhanced optical characteristics of the hybrid. A scanning electron microscope analysis revealed that synthesized GO scrolls achieved a length of over 20 µm with interior cavities. The GO-phosphor hybrid is extensively analyzed using Raman spectroscopy, suggesting that various Raman combination modes are activated with the appearance of a low-frequency radial breathing-like mode (RBLM) of the type observed in carbon nanotubes. All of the synthesized GO-phosphor hybrids exhibit an intense luminescent emission around 540 nm along with a broad emission at approximately 400 nm, with the intensity ratio varying with the GO-phosphor ratio. The photoluminescence emissions were gauged using Commission Internationale d'Eclairage (CIE) coordinates and at an optimum ratio. The coordinates shift to the white region of the color spectra. Our study suggests that the GO-phosphor hybrid nanoscrolls are suitable candidates for light-emitting applications.