Supercapacitors, also known as electrochemical capacitors, have shown great potential as energy storage devices; and 3D printing likewise as a manufacturing technique. This research progressively investigates combining these two technologies to fabricate 3D-printed, electrochemical double-layer capacitors (EDLCs). Small EDLCs were designed in a sandwich structure with an FDM-printed plastic frame and carbon electrodes. Inkjet printing was initially combined with FDM printing to produce a pilot sample with a silver ink current collector, however this performed poorly (Cs = 6 mF/g). Henceforth a paste extrusion system was added to the FDM printer to deposit the current collectors and electrodes, fabricating the entire device in a single continuous process. This process was progressively developed and tested, ultimately attaining specific capacitances of 200 mF/g. The fully integrated 3D printing process used to manufacture the EDLCs was a novel approach. Combining the FDM printer with a paste extruder allowed for a high degree of dimensional accuracy, as well as simplifying the production process. This aspect of the design functioned successfully, without significant faults, and proved a reliable fabrication method. The later designs used in this study provided the EDLCs extendable by incorporating connection jacks. This was to create the possibility to increase capacitance simply by connecting multiple EDLCs together. Tests of this feature showed that it worked well, with the extendable EDLCs delivering outputs very close to the theoretical maximum efficiency of the unit. Carbon conductive paint was applied as a current collector and electrode for the 3D printed EDLCs in an exploration of metal-free 3D printed supercapacitors. These metal-free EDLCs were found to provide around 60% of the specific capacitance of the best performing EDLC variant produced (silver paint current collectors with activated carbon and carbon paint mixture electrodes). Although considerable improvement is required to produce EDLC samples with comparable capacitances to existing commercial manufacturing techniques, this study lays important groundwork in this area, and has introduces effective and innovative design ideas for supercapacitors and integrated 3D printing processes.

The vigorous development of human civilization has significantly increased the energy consumption in recent years. There is a great need to use renewable energy sources to substitute the depleting traditional fossil fuels, such as crude oil, natural gas and coal. The development of low-cost and high-performance energy storage devices (ESDs) and systems have drawn great attention due to their feasibility as backup power supply and their applications in portable electronics and electric vehicles. Supercapacitors are among the most important ESDs because of their long charging-discharging cycle life, high power capability and a large operating temperature range. In this thesis, high-performance activated carbons (ACs)-based SCs have been synthesized from two biomass materials in both “bottom-up” and “top-down” patterns, including high fructose corn syrup and soybean residues, which are economic and environmental friendly. Firstly, a hydrothermal carbonization (HTC) - physical activation method is presented to synthesize activated carbons from high fructose corn syrup (HFCS). The effect of the activation time on the geometrical and porous characteristics of the ACs is investigated. The electrochemical performance of the supercapacitor cells made from AC treated at 850ºC for 4 hours are found as the best with a specific capacitance of 168 F/g at 0.2 A/g in 6 M KOH aqueous system. Secondly, a two-step HTC process followed by a physical activation to prepare activated carbons from soybean residue is presented. The effect of activation temperature on geometrical and porous characteristics of the ACs is studied. The ACs activated at 850ºC are found partly crystallized and exhibit a specific capacitance of 227 F/g at 2 mV/s. To understand the effect of mechanical deformation of the electrode materials on the electrochemical performance of electrical double-layer supercapacitors, a series of compression tests of HFCS-based ACs are further conducted in both dry and wet conditions. The nominal stiffness of the compressed ACs is calculated from the unloading curves. For both dry and wet disks the stiffness get increased with increased compression load, where the wet ones get higher stiffness than that of the dry ones. A simple model of porous materials is used to explain the increase in the stiffness of a compressed disk with the increase of pressure. Lastly, the effect of mechanical deformation on the electrochemical impedance of HFCS-based ACs is studied. When increasing the mechanical pressure from 4 to 81.5 KPa, the system resistance shows a relatively stable trend around 1 ohm, while the charge transfer resistance shows a dramatic dependence on mechanical pressure decreasing from 420 ohms to 1.5 ohms.

Over the past few years, energy storage devices have received tremendous interest due to the increasing demand for sustainable and renewable energy in modern society. Supercapacitors are considered as one of the most promising energy storage devices because of their high power density and long cycle life. However, low energy density remains as the main shortcoming for supercapacitors. The overall performance of supercapacitors is predominantly determined by the characteristics of the electrodes. Specifically, constructing nanostructured electrode material has been proven as an efficient way to improve the performance by providing large surface area and short ionic and electronic diffusion paths. Another approach to improve the performance of supercapacitors is the rational design of the asymmetric supercapacitor (ASC), which can extend the operation voltage. In this regard, we have focused on the synthesis and utilization of several nanomaterials, in particular, pseudocapacitance materials such as metal oxides and metal phosphides, on both positive and negative electrodes, as well as the ASC design and fabrication. First, three-dimensional TiO2 nanorod arrays have been synthesized on Ti substrate by a facile one-step hydrothermal method and demonstrated as an ideal supercapacitor positive electrode, which exhibited good areal capacitance and excellent cycling stability. Owing to the novel “dissolve and grow” mechanism, this method provides a simple and low-cost technique for flexible supercapacitor application. Second, using cobalt phosphide and iron phosphide as examples, we have demonstrated metal phosphides as high-performance supercapacitor negative electrodes for the first time. Cobalt phosphide nanowire arrays have been synthesized and presented a high capacitance of 571.3 mF/cm2. To improve the cycling stability, gel electrolyte was used to suppress the irreversible electrochemical reaction. The flexible solid-state asymmetric MnO2//CoP supercapacitor exhibited good electrochemical performance, such as a high energy density of 0.69 mWh/cm3 and a high power density of 114.2 mW/cm3. Furthermore, a PEDOT coating has been adapted to further enhance the cycling stability as well as capacitance performance of FeP nanorod arrays. The FeP/PEDOT electrode represents an outstanding capacitance of 790.59 mF/cm2 and a good stability of 82.12% retention after 5000 cycles. In addition, a MnO2//FeP/PEDOT ASC was fabricated with an excellent volumetric capacitance of 4.53 F/cm3 and an energy density of 1.61 mWh/cm3.

This research is aimed at synthesizing millimeter-scale carbon nanotube arrays (CNTA) by conventional chemical vapor deposition (CCVD) and water-assisted chemical vapor deposition (WACVD) methods, and exploring their application as catalyst supports for electrochemical supercapacitors. The growth mechanism and growth kinetics of CNTA under different conditions were systematically investigated to understand the relationship among physical characteristics of catalyst particles, growth parameters, and carbon nanotube (CNT) structures within CNTAs. Multiwalled CNT (MWCNT) array growth demonstrates lengthening and thickening stages in CCVD and WACVD. In CCVD, the lengthening and thickening were found to be competitive. By investigating catalyst particles after different pretreatment conditions, it has been found that inter-particle spacing plays a significant role in influencing CNTA height, CNT diameter and wall number. In WACVD, a long linear lengthening stage has been found. CNT wall number remains constant and catalysts preserve the activity in this stage, while MWCNTs thicken substantially and catalysts deactivate following the previously proposed radioactive decay model in the thickening stage of WACVD. Water was also shown to preserve the catalyst activity by significantly inhibiting catalyst-induced and gas phase-induced thickening processes in WACVD. Mn3O4 nanoparticles were successfully deposited and uniformly distributed within millimeter-long CNTAs by dip-casting method from non-aqueous solutions. After modification with Mn3O4 nanoparticles, CNTAs have been changed from hydrophobic to hydrophilic without their alignment and integrity being destroyed. The hydrophilic Mn3O4/CNTA composite electrodes present ideal capacitive behavior with high reversibility. This opens up a new route of utilizing ultra-long CNTAs, based on which a scalable and cost-effective method was developed to fabricate composite electrodes using millimeter-long CNTAs. To improve the performance of the composites, -MnO2 nanorods were anodically pulse-electrodeposited within hydrophilic 0.5 mm-thick Mn3O4 decorated CNTAs. The maximum gravimetric capacitance for the MnO2 nanorods/CNTA composite electrode was found to be 185 F/g, and that for -MnO2 nanorods was determined to be 221 F/g. After electrodeposition, the area-normalized capacitance and volumetric capacitance values were increased by a factor of 3, and an extremely high area-normalized capacitance of 1.80 F/cm2 was also achieved for the MnO2 nanorods/CNTA composite. / Materials Engineering

Well dispersed aqueous suspensions containing single-walled carbon nanotubes (SWCNTs) from bulk powders were prepared with surfactant and without surfactant by acid functionalization. SWCNT coated electrodes were then prepared from the SWCNT aqueous suspensions using various methods to create uniform nanoporous networks of SWCNTs on various substrates and stainless steel (SST) current collectors for use as ultracapacitor electrodes. Drop coating, high voltage electro-spraying (HVES), inkjet printing, and electrophoretic deposition (EPD) methods were evaluated. Optical and scanning electron microscope images were used to evaluate the SWCNT dispersion quality of the various electrodes. Ultimately an EPD process was established which reliably produced uniform SWCNT nanoporous networks on SST substrates. The prepared SWCNT coated electrodes were characterized using cyclic voltammetry and their capacitance was determined. A correlation between extended EPD processing times, EPD processing temperatures, and electrode capacitance was quantified. Optimum EPD processing occurs where linear capacitance gains were observed for processing times less than 10 minutes. At processing times between 10 – 60 minutes a non-linear relationship demonstrated diminishing capacitance gains with extended EPD processing times. Likewise, optimum EPD processing occurs when the processing temperature of the SWCNT suspension is raised above room temperature. At processing temperatures from 45°C to 60°C an increase in capacitance was observed over the room temperature (22°C) electrodes processed for the same durations. Conversely, for processing temperatures less than room temperature, at 5°C, a decrease in capacitance was observed. It was also observed that SWCNT electrodes processed at 60°C processing temperatures resulted in 4 times the capacitance of 5°C electrodes for the same processing times, when the durations were 8 minutes or less. For samples with raised processing temperatures the time dependent capacitance gains were observed to be significantly diminished beyond 10 minute processing times. The SWCNT network thickness was also correlated to EPD processing temperature and capacitance. A linear relationship was identified between the SWCNT network thickness and the capacitance of the electrode. It was also observed that elevated processing temperatures increase the EPD deposition rate of SWCNTs, produce thicker SWCNT networks, and thus create electrodes with higher capacitance than electrodes processed at lower EPD processing temperatures. EPD of SWCNTs was demonstrated in this work to be an effective method for the fabrication of SWCNT ultracapacitor electrodes. Characterization of the process determined that optimal EPD processing occurs within the first 10 minutes of processing time and that elevated processing temperatures yield higher SWCNT deposition rates and higher capacitance values. In this work the addition of SWCNT nanoporous networks to SST electrodes resulted in increases in capacitance of up to 398 times the capacitance of the uncoated SST electrodes yielding a single 1cm2 electrode with a capacitance of 91mF and representing an estimated specific capacitance for the processed SWCNT material of 45.78F/g.

Barium Strontium Titanate (BST) based ferroelectric thin film devices have been popular over the last decade due to their versatile applications in tunable microwave devices such as delay lines, resonators, phase shifters, and varactors. BST thin films are promising candidates due to their high dielectric constant, tunability and low dielectric loss. Dielectric-tunable properties of BST films deposited by different deposition techniques have been reported which study the effects of factors, such as oxygen vacancies, film thickness, grain size, Ba/Sr ratio, etc. Researchers have also studied doping concentrations, high temperature annealing and multilayer structures to attain higher tunability and lower loss. The aim of this investigation was to study material properties of Barium Strontium Titanate from a comprehensive point of view to establish relations between various growth techniques and the film physical and electrical properties. The primary goal of this investigation was to synthesize and characterize RF magnetron sputtered Barium Strontium Titanate (Ba1-xSrxTiO3), thin film structures and compare their properties with BST thin films deposited by sol-gel method with the aim of determining relationships between the oxide deposition parameters, the film structure, and the electric field dependence. In order to achieve higher thickness and ease of fabrication, and faster turn around time, a `stacked' deposition process was adopted, wherein a thin film (around 200nm) of BST was first deposited by RF magnetron sputtering process followed by a sol-gel deposition process to achieve higher thickness. The investigation intends to bridge the knowledge gap associated with the dependence of thickness variation with respect to the tunability of the films. The film structures obtained using the three different deposition methods were also compared with respect to their analytical and electrical properties. The interfacial effect on these `stacked' films that enhance the properties, before and after annealing these structures was also studied. There has been significant attention given to Graphene-based supercapacitors in the last few years. Even though, supercapacitors are known to have excellent energy storage capability, they suffer from limitations pertaining to both cost and performance. Carbon (CNTs), graphene (G) and carbon-based nanocomposites, conducting polymers (polyaniline (PANI), polypyrrole (PPy), etc.) have been the fore-runners for the manufacture of supercapacitor electrodes. In an attempt to better understand the leakage behavior of Graphene Polyaniline (GPANI) electrodes, BST and BST thin films were incorporated as constituents in the process of making supercapacitor electrodes resulting in improved leakage behavior of the electrochemical cells. A detailed physical, chemical and electrochemical study of these electrochemical cells was performed. The BST thin films deposited were structurally characterized using Veeco Dektek thickness profilometer, X-ray diffraction (XRD), Scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The interfacial structural characterization was carried out using high-resolution transmission electron microscopy (HRTEM). This investigation, also presents noncontact electrical characterization of BST films using Corona Kelvin metrology (C-KM). The `stacked' BST thin films and devices, which were electrically tested using Corona Kelvin metrology, showed marked improvement in their leakage characteristics over both, the sputtered and the sol-gel deposited counterparts. The `stacked' BST thin film samples were able to withstand voltages up to 30V positive and negative whereas, the sol-gel and sputtered samples could hold only up to a few volts without charge leaking to reduce the overall potential. High frequency, 1GHz, studies carried out on BST thin film interdigitated capacitors yielded tunability near 43%. Leakage barrier studies demonstrated improvement in the charging discharging response of the GPANI electrochemical electrodes by 40% due to the addition of BST layer.

No description available.

Layer-by-Layer (LbL) deposition of electrochemically active materials on porous carbon electrodes is a proven method to leverage both electrochemical double-layer capacitance and pseudocapacitance for charge storage on the same electrode. LbL coatings are held together by electrostatic attraction between adjacent layers of oppositely charged molecules. Previous studies have used Keggin polyoxometalates to great effect as the anionic layer in LbL electrode modification, but little effort has been devoted to cationic material selection and LbL process optimization. This work investigated alternatives to the conventional, electrochemically inert polydiallyldimethylammonium (PDDA) cation. The use of fuchsin molecular cations in LbL deposition improved the specific energy and specific power of modified electrodes. Fuchsin cation also rendered the environmentally harmful oxidative surface activation step unnecessary for LbL deposition. Process parameters were optimized for MWCNT/Fuchsin/POM samples, and post-LbL electrochemical polymerization was found to further improve the performance of these electrodes.

Electrochemical capacitors, now often termed supercapacitors, are high power electrochemical energy storage devices that complement or replace high power batteries in applications ranging from wind turbines to hybrid engines to uninterruptable power supplies to electronic devices. My dissertation explores the applications of relatively uncommon techniques for both supercapacitor material syntheses and gaining better mechanistic understanding of factors impacting electrochemical performance of supercapacitors. From fundamental ion electroadsorption studies made possible by using small angle neutron scattering (SANS), to the systematic investigations of coating thickness and microstructure in metal oxide / carbon nanocomposite electrodes realized through the novel use of the atomic layer deposition (ALD) technique, new avenues of material characterization and fabrication have been studied. In this dissertation I first present the motivation to expand the knowledge of supercapacitor science and technology, and follow with an in-depth literature review of the state of the art. The literature review covers different types of supercapacitors, the materials used in the construction of commercial and exploratory devices, an exploration of the numerous factors which affect supercapacitor performance, and an overview of relevant materials synthesis and characterization techniques The technical objectives for the work performed in this dissertation are then presented, followed by the contributions that I made in this field in my two primary research thrusts: advances to the understanding of ion electroadsorption theory in both aqueous and organic electrolytes through the development of a SANS-based methodology, and advances to metal-oxide carbon nanocomposites as electrodes through the use of ALD. The understanding of ion electro-adsorption on the surface of microporous (pores < 2 nm) solids is largely hindered by the lack of experimental techniques capable of identifying the sites of ion adsorption and the concentration of ions at the nanoscale. In the first research thrust of my dissertation, I harness the high penetrating power and sensitivity of neutron scattering to isotope substitution to directly observe changes in the ion concentration as a function of the applied potential and the pore size. I have conducted initial studies in selected aqueous and organic electrolytes and outlined the guidelines for conducting such experiments for the broad range of electrode-ions-solvent combinations. I unambiguously demonstrate that depending on the solvent properties and the solvent-pore wall interactions, either enhanced or reduced ion electro-adsorption may take place in sub-nanometer pores. More importantly, for the first time I demonstrate the route to identify the critical pore size below which either enhanced or reduced electrosorption of ions takes place. My studies experimentally demonstrate that poor electrolyte wetting in the smallest pores may indeed limit device performance. The proposed methodology opens new avenues for systematic in-situ studies of complex structure-property relationships governing adsorption of ions under applied potential, critical for rational optimization of device performance. In addition to enhancing our understanding of ion sorption, there is a critical need to develop novel supercapacitor electrode materials with improved high-energy and high-power characteristics. The formation of carbon-transition metal oxide nanocomposites may offer unique benefits for such applications. Broadly available transition metal oxides, such as vanadium oxide, offer high ion storage capabilities due to the broad range of their oxidation states, but suffer from high resistivities. Carbon nanomaterials, such as carbon nanotubes (CNT), in contrast are not capable to store high ion content, but offer high and readily accessible surface area and high electrical conductivity. In the second research thrust of my thesis, by exploiting the ability of atomic layer deposition (ALD) to produce uniform coatings of metal oxides on CNT electrodes, I demonstrated an effective way to produce high power supercapacitor electrodes with ultra-high energy capability. The electrodes I developed showed stable performance with excellent capacitance retention at high current densities and sweep rates. Electrochemical performance of the oxide layers were found to strongly depend on the coating thickness. Decreasing the vanadium oxide coating thickness to ~10 nm resulted in some of the highest values of capacitance reported to date (~1550 F·g⁻¹VOx at 1 A·g⁻¹ current density). Similar methodology was utilized for the deposition of thin vanadium oxide coatings on other substrates, such as aluminum (Al) nanowires. In this case the VOₓ coated Al nanowire electrodes with 30-50% of the pore volume available for electrolyte access show volumetric capacitance of 1390-1950 F cc⁻¹, which exceeds the volumetric capacitance of porous carbons and many carbon-metal oxide composites by more than an order of magnitude. These results indicated the importance of electrode uniformity and precise control over conformity and thickness for the optimization of supercapacitor electrodes.

Well dispersed aqueous suspensions containing single-walled carbon nanotubes (SWCNTs) from bulk powders were prepared with surfactant and without surfactant by acid functionalization. SWCNT coated electrodes were then prepared from the SWCNT aqueous suspensions using various methods to create uniform nanoporous networks of SWCNTs on various substrates and stainless steel (SST) current collectors for use as ultracapacitor electrodes. Drop coating, high voltage electro-spraying (HVES), inkjet printing, and electrophoretic deposition (EPD) methods were evaluated. Optical and scanning electron microscope images were used to evaluate the SWCNT dispersion quality of the various electrodes. Ultimately an EPD process was established which reliably produced uniform SWCNT nanoporous networks on SST substrates. The prepared SWCNT coated electrodes were characterized using cyclic voltammetry and their capacitance was determined. A correlation between extended EPD processing times, EPD processing temperatures, and electrode capacitance was quantified. Optimum EPD processing occurs where linear capacitance gains were observed for processing times less than 10 minutes. At processing times between 10 – 60 minutes a non-linear relationship demonstrated diminishing capacitance gains with extended EPD processing times. Likewise, optimum EPD processing occurs when the processing temperature of the SWCNT suspension is raised above room temperature. At processing temperatures from 45°C to 60°C an increase in capacitance was observed over the room temperature (22°C) electrodes processed for the same durations. Conversely, for processing temperatures less than room temperature, at 5°C, a decrease in capacitance was observed. It was also observed that SWCNT electrodes processed at 60°C processing temperatures resulted in 4 times the capacitance of 5°C electrodes for the same processing times, when the durations were 8 minutes or less. For samples with raised processing temperatures the time dependent capacitance gains were observed to be significantly diminished beyond 10 minute processing times. The SWCNT network thickness was also correlated to EPD processing temperature and capacitance. A linear relationship was identified between the SWCNT network thickness and the capacitance of the electrode. It was also observed that elevated processing temperatures increase the EPD deposition rate of SWCNTs, produce thicker SWCNT networks, and thus create electrodes with higher capacitance than electrodes processed at lower EPD processing temperatures. EPD of SWCNTs was demonstrated in this work to be an effective method for the fabrication of SWCNT ultracapacitor electrodes. Characterization of the process determined that optimal EPD processing occurs within the first 10 minutes of processing time and that elevated processing temperatures yield higher SWCNT deposition rates and higher capacitance values. In this work the addition of SWCNT nanoporous networks to SST electrodes resulted in increases in capacitance of up to 398 times the capacitance of the uncoated SST electrodes yielding a single 1cm2 electrode with a capacitance of 91mF and representing an estimated specific capacitance for the processed SWCNT material of 45.78F/g.