This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

Almost every form of alternative energy and energy system being implemented today, e.g., wind, solar, hybrid electric and hydrogen fuel cell vehicles, depends on electrical energy storage (EES). At the DOE basic energy sciences workshop on Basic Research Needs for Electrical Energy Storage held April, 2007, the conclusion was reached that "revolutionary breakthroughs in EES are perhaps the most crucial need for this nation’s secure energy future." The workshop, chaired by John Goodenough, University of Texas-Austin, focused on the two primary methods of EES - batteries and electrochemical double-layer capacitors (also referred to as ‘ultracapacitors’ and ‘supercapacitors’). As stated in the report from this DOE workshop, “The performance of current EES technologies falls well short of requirements for using electrical energy efficiently in transportation, commercial, and residential applications.” In this dissertation, increasing the energy storage capacity of ultracapacitors through the use of graphene electrode materials is investigated. Chapter 1 is a basic overview of EES applications and ultracapacitor technology. In Chapter 2, best practice experimental procedures to accurately evaluate a material’s performance are described. Because current measurement methods for determining a material’s performance for use as an ultracapacitor electrode are not well standardized, the different techniques currently being employed lead to wide variations in reported results. Reliable methods that would accurately test a large number of samples involving minute quantities of material were required. In Chapter 3, the performance of graphene-derived materials is investigated. Chemically modified graphene materials gave values competitive with current activated carbons and an ultracapacitor based on activated graphene electrodes yielded the highest specific capacitance values reported to date. Chapter 4 describes a lithium ion hybrid supercapacitor using this novel material that gave energy densities greater than lead acid batteries. The exceptional performance of these graphene derived materials will likely result in their rapid adoption as well as an expanded range of applications utilizing ultracapacitors. As increasingly higher surface area graphene materials are developed, a fundamental understanding of the components that affect interfacial capacitance is critical for further capacity increases. In the last chapter, the first direct measurement of the interfacial capacitance for one and two sides of single layer graphene is presented. The results show that the quantum capacitance increasingly becomes a factor with the result being a reduced increase in capacitance, not the linear increase with surface area as would be expected for bulk conductive materials. These results indicate that the development of higher surface area graphene materials alone is not sufficient for additional increases in performance; the modification of the electronic properties will also be required. / text

This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

Microbial  Cellulose  (MC)  is  a highly  porous  macromolecule  with  intrinsic  properties  that  make  it  a  useful  substrate  for  conductive  materials  within  ultracapacitors.  MC  has  the  potential  to  increase  capacitance  by  serving  as  a  high  surface  area  substrate  for  conductive  polymers  and  carbonaceous  materials.  Electrode  surface  area  is  a  critical  parameter  in  ultracapacitors  because  capacitance  depends  on  the  available  active  sites  that  are  accessible  to  counter  ions.  Commercial  ultracapacitors  increase  electrode  surface  area  by  adding  micro­size  carbonaceous  materials.  Most  commercial  devices  also  require  adhesive  compounds  to  bind  the  conductive  material  to  the  substrate.  Adhesive  compounds  increase  sheet  resistance  and  hinder  overall  capacitance.  MC  membranes  possess  highly­ordered  surface  hydroxyl  groups  that  readily  bind  to  different  types  conductive  materials  and  reduce  the  need  for  additive  adhesive  compounds.  This  thesis  investigates  three  unique  methods  for  converting  a  MC  membrane  into  a  working  ultracapacitor electrode. In  the  first  method,  polypyrrole  and  carbon  nanotubes  (CNTs)  are  added  to  a  medium  of  Acetobacter  that  incorporates  the  material  into  a  homogeneous  crystalline  matrix of beta­1,4 glucan chains. The resulting MC is a fully integrated membrane  with a  homogeneous  embedded  layer  of  conductive  material.  SEM  imaging  shows  the  conductive  material  is  incorporated  primarily  at  the  core  of  the  membrane.  As  a  result,  this  electrode  suffered  from  high  sheet  resistance  and  did  not  generate  any  significant  capacitance.  In  the  second  method,  a  conductive  ink  consisting  of  CNTs,  carboxymethyl  cellulose  (CMC),  polypyrrole,  and  DI  water  was  used  to  coat  the  surface  of  a  dried  cellulose  membrane.  After  1­2  hours,  the  ink  dries  and  leaves  a  shiny  black  conductive  layer  on  the  membrane’s  surface.  CMC’s  role  in  the  ink  is  to  increase  viscosity  and  help  bind  the  conductive  material  to  the  membrane  surface.  CMC  is  also  a  dielectric  material  that  acts  as  an  insulator  to  the  polypyrrole  and  CNTs,  and  ultimately  impedes  electrical  energy  storage.  In  the  final  method,  a  MC  membrane  was  soaked  in  aqueous  and  non­ aqueous  pyrrole  solutions,  and  polymerized  with  FeCl3  and  Fe2(SO4)3.  Single  and  double  membrane  device  configurations  were  also  investigated.  Surface  polymerization  of  pyrrole  monomers  proved  to  be  the  best  method  for  converting  microbial  cellulose  into  a  working electrode with good capacitance and cyclability. / text

Electrochemical capacitors or supercapacitors or ultracapacitors are potential energy storage devices that could help bringing major advances in future energy storage applications. Unlike batteries that store energy in chemical reactants capable of generating charge, electrochemical capacitors store energy directly through charge separation. Most electrochemical capacitors rely on carbon-based structures utilizing electrical double-layer capacitance effect. By contrast, a pseudocapacitor relies on charge stored due to fast faradaic charge-transfer processes with surface atoms. A combination of faradaic and non-faradaic components would generate hybrid electrochemical capacitors or hybrid ultracapacitors that attain high capacitance for pulse power and sustained energy. This thesis comprises studies pertaining to design, development and applications R&D on substrate-integrated lead-carbon hybrid ultracapacitors. The thesis comprises ten chapters. Chapter 1 is a brief introduction on essentials of electrochemical capacitors explaining their operating principles, classification and applications. Chapter 2 describes studies on materials for electrical double-layer capacitors. Activated carbons are the most common materials for electrical double-layer capacitors. Various activated carbon samples are screened as suitable materials for electrical double-layer capacitor followed by their optimization under varying experimental conditions to form the negative plate in the substrate-integrated lead-carbon hybrid ultracapacitor. Chapter 3 deals with the studies on design and development of 2 V substrate-integrated lead-carbon hybrid ultracapacitors with flooded, absorbent-glass-mat and silica-gel sulfuric acid electrolyte configurations. Lead-carbon hybrid ultracapacitors comprise substrate-integrated lead dioxide sheets as positive plates and high surface-area-carbon-coated graphite-sheets as negative plates. Operating principle for 2 V lead-carbon hybrid ultracapacitors is explained and optimization of their operating conditions along with their electrochemical performance is studied. Chapter 4 is a study on the integration of 2 V substrate-integrated lead-carbon hybrid ultracapacitors to 12 V devices. 12 V substrate-integrated lead-carbon hybrid ultracapacitors with flooded, absorbent-glass-mat and silica gel sulfuric acid electrolyte are developed by connecting six 2 V cells in series. These hybrid ultracapacitors exhibit high power-density values and excellent cycle-life. The problem of uneven performance among the six 2 V cells in the 12 V hybrid ultracapacitors is addressed and resolved by applying voltage-management cell-balancing circuitry. Chapter 5 details the studies on kilo-Farad range 12 V substrate-integrated lead-carbon hybrid ultracapacitors. The hybrid ultracapacitors are performance tested through a standard protocol. Thermal runaway in these hybrid ultracapacitors at high load currents is studied by thermal imaging. Studies on performance comparison between 12 V lead-carbon hybrid ultracapacitors with substrate-integrated and conventional pasted-positive plates are presented in Chapter 6. For substrate-integrated-positive plate lead-carbon hybrid ultracapacitors, capacitance and energy-density values are lower but power-density values are higher than pasted-positive plate configuration due to their shorter response-time. Accordingly, internal resistance values are lower for substrate-integrated lead-carbon hybrid ultracapacitors. Both types of lead-carbon hybrid ultracapacitors exhibit similar faradaic efficiency and cycle-life in excess of 100,000 pulse charge/discharge cycles with only a nominal loss in their capacitance values. Chapter 7 is a study on the design and development of low-cost substrate-integrated lead-carbon hybrid ultracapacitors using poly-aniline organic metal. The hybrid ultracapacitor employs flexible exfoliated graphite sheets as negative plate current-collectors, which are coated with a thin layer of poly-aniline to provide good adhesivity to activated carbon layer and good substrate-conductivity. These ultracapacitors are estimated to cost about 4 US$/Wh as compared to 20-30 US$/Wh for presently available commercial ultracapacitors. In Chapter 8, an application R&D study on the suitability of a substrate-integrated lead-carbon hybrid ultracapacitor bank in powering medical gadgets is described. A practical application that provides 30 W power back-up to medical gadgets for use in grid-power-deficient rural areas is presented. Chapter 9 is another application R&D study in realizing a photovoltaic stand-alone lighting system using substrate-integrated lead-carbon hybrid ultracapacitors. At present, harnessing solar electricity generated through photovoltaic cells with lead-acid batteries remains the most compelling option. But lead-acid batteries have encountered problems in photovoltaic installations, mainly due to their premature failure. To circumvent this problem, substrate-integrated lead-carbon hybrid ultracapacitors are developed for solar energy storage for a lighting application. The last Chapter of the thesis comprises field studies on substrate-integrated lead-carbon hybrid ultracapacitors. In the study, hybrid ultracapacitors are installed for lighting applications for field tests. Grid-power chargers and mechanical dynamos are introduced as fast-charging tools for hybrid ultracapacitors. It is hoped that the studies presented in this thesis would constitute a worthwhile contribution to science and technology of electrochemical capacitors. Considering the technology need, availability, safety and cost, substrate-integrated lead-carbon hybrid ultracapacitors are set to play a seminal role in future energy storage and management.

No description available.

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.

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.

The behavior of 2D materials has become of great interest in the wake of development of electrochemical double-layer capacitors (EDLCs) and the discovery of monolayer graphene by Geim and Novoselov. This study aims to analyze the response variance of 2D electrode materials for EDLCs prepared through the liquid-phase exfoliation method when subjected to differing conditions. Once exfoliated, samples are tested with a series of structural characterization methods, including tunneling electron microscopy, atomic force microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. A new ionic liquid for EDLC use, 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate is compared in performance to 6M potassium hydroxide aqueous electrolyte. Devices composed of liquid-phase exfoliated graphene / MoS2 composites are analyzed by concentration for ideal performance. Device performance under cold extreme temperatures for the ionic fluid is presented as well. A brief overview of by-layer analysis of graphene electrode materials is presented as-is. All samples were tested with cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy, with good capacitive results. The evolution of electrochemical behavior through the altered parameters is tracked as well.

Over-the-air programming systems for wireless sensor networks have drawbacks that stem from fundamental limitations in the hardware used in current sensor nodes. Also, advances in technology make it feasible to use capacitors as the sole energy storage mechanism for sensor nodes using energy harvesting, but most current designs require additional electronics. These two considerations led to the design of a new sensor node. A microcontroller was chosen that meets the Popek and Goldberg virtualization requirements. The hardware design for this new sensor node is presented, as well as a preliminary operating system. The prototypes are tested, and demonstrated to be sustainable with a capacitor and solar panel. The issue of capacitor leakage is considered and measured.