Hemtej Gullapalli

Synthesis Of Nitrogen-Doped Graphene Films For Lithium Battery Application

We demonstrate a controlled growth of nitrogen-doped graphene layers by liquid precursor based chemical vapor deposition (CVD) technique. Nitrogen-doped graphene was grown directly on Cu current collectors and studied for its reversible Li-ion intercalation properties. Reversible discharge capacity of N-doped graphene is almost double compared to pristine graphene due to the large number of surface defects induced due to N-doping. All the graphene films were characterized by Raman spectroscopy, transmission electron microscopy, and X-ray photoemission spectroscopy. Direct growth of active electrode material on current collector substrates makes this a feasible and efficient process for integration into current battery manufacture technology.

Flexible Piezoelectric ZnO–Paper Nanocomposite Strain Sensor

The fabrication of a mechanically flexible, piezoelectric nanocomposite material for strain sensing applications is reported. Nanocomposite material consisting of zinc oxide (ZnO) nanostructures embedded in a stable matrix of paper (cellulose fibers) is prepared by a solvothermal method. The applicability of this material as a strain sensor is demonstrated by studying its real-time current response under both static and dynamic mechanical loading. The material presented highlights a novel approach to introduce flexibility into strain sensors by embedding crystalline piezoelectric material in a flexible cellulose-based secondary matrix.

High sensitivity detection of NO2 and NH3 in air using chemical vapor deposition grown graphene

We show that graphene films synthesized by chemical-vapor-deposition enables detection of trace amounts of nitrogen dioxide (NO2) and ammonia (NH3) in air at room temperature and atmospheric pressure. The gas species are detected by monitoring changes in electrical resistance of the graphene film due to gas adsorption. The sensor response time was inversely proportional to the gas concentration. Heating the film expelled chemisorbed molecules from the graphene surface enabling reversible operation. The detection limits of ∼100 parts-per-billion (ppb) for NO2 and ∼500 ppb for NH3 obtained using our device are markedly superior to commercially available NO2 and NH3 detectors.

Three-Dimensionally Engineered Porous Silicon Electrodes for Li Ion Batteries

The ultimate goal of Li ion battery design should consist of fully accessible metallic current collectors, possibly of nanoscale dimensions, intimately in contact with high capacity stable electrode materials. Here we engineer three-dimensional porous nickel based current collector coated conformally with layers of silicon, which typically suffers from poor cycle life, to form high-capacity electrodes. These binder/conductive additive free silicon electrodes show excellent electrode adhesion resulting in superior cyclic stability and rate capability. The nickel current collector design also allows for an increase in silicon loading per unit area leading to high areal discharge capacities of up to 0.8 mAh/cm(2) without significant loss in rate capability. An excellent electrode utilization (∼85%) and improved cyclic stability for the metal/silicon system is attributed to reduced internal stresses/fracture upon electrode expansion during cycling and shorter ionic/electronic diffusion pathways that help in improving the rate capability of thicker silicon layers.

Supercapacitor Operating At 200 Degrees Celsius

The operating temperatures of current electrochemical energy storage devices are limited due to electrolyte degradation and separator instability at higher temperatures. Here we demonstrate that a tailored mixture of materials can facilitate operation of supercapacitors at record temperatures, as high as 200°C. Composite electrolyte/separator structures made from naturally occurring clay and room temperature ionic liquids, with graphitic carbon electrodes, show stable supercapacitor performance at 200°C with good cyclic stability. Free standing films of such high temperature composite electrolyte systems can become versatile functional membranes in several high temperature energy conversion and storage applications.

Flexible ZnO-cellulose nanocomposite for multisource energy conversion

Materials with the ability to harness multiple sources of energy from the ambient environment could lead to new types of energy-harvesting systems. It is demonstrated that nanocomposite films consisting of zinc oxide nanostructures embedded in a common paper matrix can be directly used as energy-conversion devices to transform mechanical and thermal energies to electric power. These mechanically robust and flexible devices can be fabricated over large areas and are capable of producing an output voltage and power up to 80 mV and 50 nW cm(-2) , respectively. Furthermore, it is shown that by integrating a certain number of devices (in series and parallel) the output voltage and the concomitant output power can be significantly increased. Also, the output voltage and power can be enhanced by scaling the size of the device. This multisource energy-harvesting system based on ZnO nanostructures embedded in a flexible paper matrix provides a simplified and cost-effective platform for capturing trace amounts of energy for practical applications.

Graphene Growth via Carburization of Stainless Steel and Application in Energy Storage

A modified version of the carburization process, a widely established technique used in the steel industry for case hardening of components, is used for the growth of graphene on stainless steel. Controlled growth of high-quality single- and few-layered graphene on stainless steel (SS) foils through a liquid-phase chemical vapor deposition (CVD) technique is reported. Reversible Li intercalation in these graphene-on-SS structures is demonstrated, where graphene and SS act as electrode and current collector, respectively, providing very good electrical contact. Direct growth of an active electrode material, such as graphene, on current-collector substrates makes this a feasible and efficient process for developing thin-film battery devices.

Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries.

Rechargeable batteries capable of operating at high temperatures have significant use in various targeted applications. Expanding the thermal stability of current lithium ion batteries requires replacing the electrolyte and separators with stable alternatives. Since solid-state electrolytes do not have a good electrode interface, we report here the development of a new class of quasi-solid-state electrolytes, which have the structural stability of a solid and the wettability of a liquid. Microflakes of clay particles drenched in a solution of lithiated room temperature ionic liquid forming a quasi-solid system has been demonstrated to have structural stability until 355 °C. With an ionic conductivity of ∼3.35 mS cm(-1), the composite electrolyte has been shown to deliver stable electrochemical performance at 120 °C, and a rechargeable lithium battery with Li4Ti5O12 electrode has been tested to deliver reliable capacity for over several cycles of charge-discharge.

Increased mobility for layer-by-layer transferred chemical vapor deposited graphene/boron-nitride thin films

Large-area chemical vapor deposited graphene/boron-nitride (G/BN) thin films are co-transferred layer-by-layer to silicon-di-Oxide (SiO2) substrates, and transistors are constructed and examined. Raman spectra and high resolution transmission electron microscopy imaging show films of high quality. The graphene/boron-nitride/SiO2 devices have a significantly increased peak electron/hole mobility of 3400/2200 cm2/Vs with a reduced effective doping density over reference graphene/SiO2 devices. The mobility dependence as a function of carrier density is compared with a physically based empirical model and is in agreement with the improvements due to a consistent reduction in the substrate induced phonon and impurity scattering and an improvement in the overall surface quality owed to the boron-nitride interlayer that separates the graphene from the SiO2. Large-area G/BN thin films are promising for future high performance thin film electronic devices.

Low-Cost, Large-Area, Facile, and Rapid Fabrication of Aligned ZnO Nanowire Device Arrays

Well aligned nanowires of ZnO have been made with an electrospinning technique using zinc acetate precursor solutions. Employment of two connected parallel collector plates with a separating gap of 4 cm resulted in a very high degree of nanowire alignment. By adjusting the process parameters, the deposition density of the wires could be controlled. Field effect transistors were prepared by depositing wires between two gold electrodes on top of a heavily doped Si substrate covered with a 300 nm oxide layer. These devices showed good FET characteristics and photosensitivity under UV-illumination. The method provides a fast and scalable fabrication route for functional nanowire arrays with a high degree of alignment and control over nanowire spacing.