Emery Brown

Controlling dielectric and relaxor-ferroelectric properties for energy storage by tuning Pb0.92La0.08Zr0.52Ti0.48O3 film thickness.

The energy storage properties of Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films grown via pulsed laser deposition were evaluated at variable film thickness of 125, 250, 500, and 1000 nm. These films show high dielectric permittivity up to ∼1200. Cyclic I-V measurements were used to evaluate the dielectric properties of these thin films, which not only provide the total electric displacement, but also separate contributions from each of the relevant components including electric conductivity (D1), dielectric capacitance (D2), and relaxor-ferroelectric domain switching polarization (P). The results show that, as the film thickness increases, the material transits from a linear dielectric to nonlinear relaxor-ferroelectric. While the energy storage per volume increases with the film thickness, the energy storage efficiency drops from ∼80% to ∼30%. The PLZT films can be optimized for different energy storage applications by tuning the film thickness to optimize between the linear and nonlinear dielectric properties and energy storage efficiency.

Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium Pentoxide for Enhanced Performance in Lithium-Ion Batteries and Electrochemical Capacitors

Mesoporous hybrids of V2O5 nanoparticles anchored on reduced graphene oxide (rGO) have been synthesized by slow hydrolysis of vanadium oxytriisopropoxide using a two-step solvothermal method followed by vacuum annealing. The hybrid material possesses a hierarchical structure with 20-30 nm V2O5 nanoparticles uniformly grown on rGO nanosheets, leading to a high surface area with mesoscale porosity. Such hybrid materials present significantly improved electronic conductivity and fast electrolyte ion diffusion, which synergistically enhance the electrical energy storage performance. Symmetrical electrochemical capacitors with two rGO-V2O5 hybrid electrodes show excellent cycling stability, good rate capability, and a high specific capacitance up to ∼466 F g(-1) (regarding the total mass of V2O5) in a neutral aqueous electrolyte (1.0 M Na2SO4). When used as the cathode in lithium-ion batteries, the rGO-V2O5 hybrid demonstrates excellent cycling stability and power capability, able to deliver a specific capacity of 295, 220, and 132 mAh g(-1) (regarding the mass of V2O5) at a rate of C/9, 1C, and 10C, respectively. The value at C/9 rate matches the full theoretical capacity of V2O5 for reversible 2 Li(+) insertion/extraction between 4.0 and 2.0 V (vs Li/Li(+)). It retains ∼83% of the discharge capacity after 150 cycles at 1C rate, with only 0.12% decrease per cycle. The enhanced performance in electrical energy storage reveals the effectiveness of rGO as the structure template and more conductive electron pathway in the hybrid material to overcome the intrinsic limits of single-phase V2O5 materials.

Atomic layer deposition of Al-doped ZnO/Al2O3 double layers on vertically aligned carbon nanofiber arrays.

High-aspect-ratio, vertically aligned carbon nanofibers (VACNFs) were conformally coated with aluminum oxide (Al2O3) and aluminum-doped zinc oxide (AZO) using atomic layer deposition (ALD) in order to produce a three-dimensional array of metal-insulator-metal core-shell nanostructures. Prefunctionalization before ALD, as required for initiating covalent bonding on a carbon nanotube surface, was eliminated on VACNFs due to the graphitic edges along the surface of each CNF. The graphitic edges provided ideal nucleation sites under sequential exposures of H2O and trimethylaluminum to form an Al2O3 coating up to 20 nm in thickness. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirmed the conformal core-shell AZO/Al2O3/CNF structures while energy-dispersive X-ray spectroscopy verified the elemental composition of the different layers. HRTEM selected area electron diffraction revealed that the as-made Al2O3 by ALD at 200 °C was amorphous, and then, after annealing in air at 450 °C for 30 min, was converted to polycrystalline form. Nevertheless, comparable dielectric constants of 9.3 were obtained in both cases by cyclic voltammetry at a scan rate of 1000 V/s. The conformal core-shell AZO/Al2O3/VACNF array structure demonstrated in this work provides a promising three-dimensional architecture toward applications of solid-state capacitors with large surface area having a thin, leak-free dielectric.

3D Printing Hierarchical Silver Nanowire Aerogel with Highly Compressive Resilience and Tensile Elongation through Tunable Poisson's Ratio

Metallic aerogels have attracted intense attention due to their superior properties, such as high electrical conductivity, ultralow densities, and large specific surface area. The preparation of metal aerogels with high efficiency and controllability remains challenge. A 3D freeze assembling printing technique integrated with drop-on-demand inkjet printing and freeze casting are proposed for metallic aerogels preparation. This technique enables tailoring both the macrostructure and microstructure of silver nanowire aerogels (SNWAs) by integrating programmable 3D printing and freeze casting, respectively. The density of the printed SNWAs is controllable, which can be down to 1.3 mg cm-3 . The ultralight SNWAs reach high electrical conductivity of 1.3 S cm-1 and exhibit excellent compressive resilience under 50% compressive strain. Remarkably, the printing methodology also enables tuning aerogel architectures with designed Poissons ratio (from negative to positive). Moreover, these aerogel architechtures with tunable Poissons ratio present highly electromechanical stability under high compressive and tensile strain (both strain up to 20% with fully recovery).