Hybrid core-shell nanowire electrodes utilizing vertically aligned carbon nanofiber arrays for high-performance energy storage

Doctor of PhilosophyDepartment of ChemistryJun LiNanostructured electrode materials for electrochemical energy storage systems have been shown to improve both rate performance and capacity retention, while allowing considerably longer cycling lifetime. The nano-architectures provide enhanced kinetics by means of larger surface area, higher porosity, better material interconnectivity, shorter diffusion lengths, and overall mechanical stability. Meanwhile, active materials that once were excluded from use due to bulk property issues are now being examined in new nanoarchitecture. Silicon was such a material, desired for its large lithium-ion storage capacity of 4,200 mAh g[superscript]-1 and low redox potential of 0.4 V vs. Li/Li[superscript]+; however, a ~300% volume expansion and increased resistivity upon lithiation limited its broader applications. In the first study, the silicon-coated vertically aligned carbon nanofiber (VACNF) array presents a unique core-shell nanowire (NW) architecture that demonstrates both good capacity and high rate performance. In follow-up, the Si-VACNFs NW electrode demonstrates enhanced power rate capabilities as it shows excellent storage capacity at high rates, attributed to the unique nanoneedle structure that high vacuum sputtering produces on the three-dimensional array. Following silicon’s success, titanium dioxide has been explored as an alternative highrate electrode material by utilizing the dual storage mechanisms of Li+ insertion and pseudocapacitance. The TiO[subscript]2-coated VACNFs shows improved electrochemical activity that delivers near theoretical capacity at larger currents due to shorter Li[superscript]+ diffusion lengths and highly effective electron transport. A unique cell is formed with the Si-coated and TiO[subscript]2-coated electrodes place counter to one another, creating the hybrid of lithium ion battery-pseudocapacitor that demonstrated both high power and high energy densities. The hybrid cell operates like a battery at lower current rates, achieving larger discharge capacity, while retaining one-third of that capacity as the current is raised by 100-fold. This showcases the VACNF arrays as a solid platform capable of assisting lithium active compounds to achieve high capacity at very high rates, comparable to modern supercapacitors. Lastly, manganese oxide is explored to demonstrate the high power rate performance that the VACNF array can provide by creating a supercapacitor that is highly effective in cycling at various high current rates, maintaining high-capacity and good cycling performance for thousands of cycles

Preparation and Characterization of TiO₂ Barrier Layers for Dye-Sensitized Solar Cells

A TiO₂ barrier layer is critical in enhancing the performance of dye-sensitized solar cells (DSSCs). Two methods to prepare the TiO₂ barrier layer on fluorine-doped tin dioxide (FTO) surface were systematically studied in order to minimize electron–hole recombination and electron backflow during photovoltaic processes of DSSCs. The film structure and materials properties were correlated with the photovoltaic characteristics and electrochemical properties. In the first approach, a porous TiO₂ layer was deposited by wet chemical treatment of the sample with TiCl₄ solution for time periods varying from 0 to 60 min. The N719 dye molecules were found to be able to insert into the porous barrier layers. The 20 min treatment formed a nonuniform but intact TiO₂ layer of ∼100–300 nm in thickness, which gave the highest open-circuit voltage <i>V</i>_OC, short-circuit photocurrent density <i>J</i>_SC, and energy conversion efficiency. But thicker TiO₂ barrier layers by this method caused a decrease in <i>J</i>_SC, possibly limited by lower electrical conductance. In the second approach, a compact TiO₂ barrier layer was created by sputter-coating 0–15 nm Ti metal films on FTO/glass and then oxidizing them into TiO₂ with thermal treatment at 500 °C in the air for 30 min. The dye molecules were found to only attach at the outer surface of the barrier layer and slightly increased with the layer thickness. These two kinds of barrier layer showed different characteristics and may be tailored for different DSSC studies

Effective Infiltration of Gel Polymer Electrolyte into Silicon-Coated Vertically Aligned Carbon Nanofibers as Anodes for Solid-State Lithium-Ion Batteries

This study demonstrates the full infiltration of gel polymer electrolyte into silicon-coated vertically aligned carbon nanofibers (Si-VACNFs), a high-capacity 3D nanostructured anode, and the electrochemical characterization of its properties as an effective electrolyte/separator for future all-solid-state lithium-ion batteries. Two fabrication methods have been employed to form a stable interface between the gel polymer electrolyte and the Si-VACNF anode. In the first method, the drop-casted gel polymer electrolyte is able to fully infiltrate into the open space between the vertically aligned core–shell nanofibers and encapsulate/stabilize each individual nanofiber in the polymer matrix. The 3D nanostructured Si-VACNF anode shows a very high capacity of 3450 mAh g^–1 at C/10.5 (or 0.36 A g^–1) rate and 1732 mAh g^–1 at 1C (or 3.8 A g^–1) rate. In the second method, a preformed gel electrolyte film is sandwiched between an Si-VACNF electrode and a Li foil to form a half-cell. Most of the vertical core–shell nanofibers of the Si-VACNF anode are able to penetrate into the gel polymer film while retaining their structural integrity. The slightly lower capacity of 2800 mAh g^–1 at C/11 rate and ∼1070 mAh g^–1 at C/1.5 (or 2.6 A g^–1) rate have been obtained, with almost no capacity fade for up to 100 cycles. Electrochemical impedance spectroscopy does not show noticeable changes after 110 cycles, further revealing the stable interface between the gel polymer electrolyte and the Si-VACNFs anode. These results show that the infiltrated flexible gel polymer electrolyte can effectively accommodate the stress/strain of the Si shell due to the large volume expansion/contraction during the charge–discharge processes, which is particularly useful for developing future flexible solid-state lithium-ion batteries incorporating Si-anodes

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

Tin Dioxide@Carbon Core–Shell Nanoarchitectures Anchored on Wrinkled Graphene for Ultrafast and Stable Lithium Storage

The SnO₂@C@GS composites as a new type of 3D nanoarchitecture have been successfully synthesized by a facile hydrothermal process followed by a sintering strategy. Such a 3D nanoarchitecture is made up of SnO₂@C core–shell nanospheres and nanochains anchored on wrinkled graphene sheets (GSs). Transmission electron microscopy shows that these core–shell nanoparticles consist of 3–9 nm diameter secondary SnO₂ nanoparticles embedded in about 50 nm diameter primary carbon nanospheres. Large quantities of core–shell nanoparticles are uniformly attached to the surface of wrinkled graphene nanosheets, with a portion of them further connected into nanochains. This new 3D nanoarchitecture consists of two different kinds of carbon-buffering matrixes, i.e., the carbon layer produced by glucose carbonization and the added GS template, leading to enhanced lithium storage properties. The lithium-cycling properties of the SnO₂@C@GS composite have been evaluated by galvanostatic discharge–charge cycling and electrochemical impedance spectroscopy. Results show that the SnO₂@C@GS composite has discharge capacities of 883.5, 845.7, and 830.5 mA h g^–1 in the 20th, 50th and 100th cycles, respectively, at a current density of 200 mA g^–1 and delivers a desirable discharge capacity of 645.2 mA h g^–1 at a rate of 1680 mA g^–1. This new 3D nanoarchitecture exhibits a high capability and excellent cycling and rate performance, holding great potential as a high-rate and stable anode material for lithium storage