Controlling Dielectric and Relaxor-Ferroelectric Properties for Energy Storage by Tuning Pb_0.92La_0.08­Zr_0.52Ti_0.48O₃ Film Thickness

The energy storage properties of Pb_0.92La_0.08­Zr_0.52Ti_0.48O₃ (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>I</i>–<i>V</i> 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 (<i>D1</i>), dielectric capacitance (<i>D2</i>), and relaxor-ferroelectric domain switching polarization (<i>P</i>). 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