Modeling of electrical behavior of graphene-based ultracapacitors

Graphene has been identified as a promising material for energy storage, especially for high performance ultracapacitors. Graphene-based ultracapacitors show high stability, significantly-improved capacitance and energy density with fast charging and discharging time at a high current density, due to enhanced ionic electrolyte accessibility in deeper regions. The surface area of a single graphene sheet is 2630 m2/g, substantially higher than values derived from Brunauer Emmett Teller (BET) surface area measurements of activated carbons used in the current electrochemical double layer capacitors. In an ultracapacitor cell, chemically modified graphene (CMG) materials demonstrate high specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively. In addition, high electrical conductivity gives these materials consistently good performance over a wide range of voltage scan rates. This paper reports a modeling methodology to predict the electrical behavior of a 2.7 V/650 F ultracapacitor cell. The ultracapacitor cell is subject to the charge/discharge cycling with constant-current between 1.35 V and 2.7 V. The charge/discharge current values examined are 50, 100, 150, and 200 A. A three resistor-capacitor (RC) parallel branch model is employed to calculate the electrical behavior of the ultracapacitor. The simulation results for the variations of the cell voltage as a function of time for various charge/discharge currents are in good agreement with the experimental measurements

Defect-engineered graphene for bulk supercapacitors with high energy and power densities

The development of high-energy and high-power density supercapacitors (SCs) is critical for enabling next-generation energy storage applications. Nanocarbons are excellent SC electrode materials due to their economic viability, high-surface area, and high stability. Although nanocarbons have high theoretical surface area and hence high double layer capacitance, the net amount of energy stored in nanocarbon-SCs is much below theoretical limits due to two inherent bottlenecks: i) their low quantum capacitance and ii) limited ion-accessible surface area. Here, we demonstrate that defects in graphene could be effectively used to mitigate these bottlenecks by drastically increasing the quantum capacitance and opening new channels to facilitate ion diffusion in otherwise closed interlayer spaces. Our results support the emergence of a new energy paradigm in SCs with 250% enhancement in double layer capacitance beyond the theoretical limit. Furthermore, we demonstrate prototype defect engineered bulk SC devices with energy densities 500% higher than state-of-the-art commercial SCs without compromising the power density.Comment: 15 pages, 5 figures, and 8 supplemental figure

Utilization of multiple graphene layers in fuel cells. 1. An improved technique for the exfoliation of graphene-based nanosheets from graphite

An improved, safer and mild method was proposed for the exfoliation of graphene like sheets from graphite to be used in fuel cells. The major aim in the proposed method is to reduce the number of layers in the graphite material and to produce large quantities of graphene bundles to be used as catalyst support in polymer electrolyte membrane fuel cells. Graphite oxide was prepared using potassium dichromate/sulfuric acid as oxidant and acetic anhydride as intercalating agent. The oxidation process seemed to create expanded and leafy structures of graphite oxide layers. Heat treatment of samples led to the thermal decomposition of acetic anhydride into carbondioxide and water vapor which further swelled the layered graphitic structure. Sonication of graphite oxide samples created more separated structures. Morphology of the sonicated graphite oxide samples exhibited expanded the layer structures and formed some tullelike translucent and crumpled graphite oxide sheets. The mild procedure applied was capable of reducing the average number of graphene sheets from 86 in the raw graphite to nine in graphene-based nanosheets. Raman spectroscopy analysis showed the significant reduction in size of the in-plane sp2 domains of graphene nanosheets obtained after the reduction of graphite oxide

Super Dielectric Materials

Evidence is provided that a class of materials with dielectric constants greater than 100,000, herein called super dielectric materials (SDM), can be generated readily from common, inexpensive materials.Comment: The first material ever with an intrinsic dielectric constant greater than 100,000. Postulated to be a class of materials with super dielectric propertie

Growth and properties of few-layer graphene prepared by chemical vapor deposition

The structure, and electrical, mechanical and optical properties of few-layer graphene (FLG) synthesized by chemical vapor deposition (CVD) on a Ni coated substrate were studied. Atomic resolution transmission electron microscope (TEM) images show highly crystalline single layer parts of the sample changing to multilayer domains where crystal boundaries are connected by chemical bonds. This suggests two different growth mechanisms. CVD and carbon segregation participate in the growth process and are responsible for the different structural formations found. Measurements of the electrical and mechanical properties on the centimeter scale provide evidence of a large scale structural continuity: 1) in the temperature dependence of the electrical conductivity, a non-zero value near 0 K indicates the metallic character of electronic transport; 2) the Young's modulus of a pristine polycarbonate film (1.37 GPa) improves significantly when covered with FLG (1.85 GPa). The latter indicates an extraordinary Young modulus value of the FLG-coating of TPa orders of magnitude. Raman and optical spectroscopy support the previous conclusions. The sample can be used as a flexible and transparent electrode and is suitable for special membranes to detect and study individual molecules in high resolution TEM

Graphene/Ionic Liquid Ultracapacitors: Does Ionic Size Correlate with Storage Energy Performance?

An electric double layer ultracapacitor stores energy in an electric double layer formed near its electrolyte/electrode interfaces. Graphene-based ultracapacitors, because of their outstanding performance, have attracted significant research interest. Optimization of ultracapacitor performance requires understanding the correlation of molecular characteristic of the device (such as structure, inter-ionic and ion-electrode interactions) with its macroscopic properties. Herein, we report molecular dynamics study of how an ionic volume impacts a double-layer capacitance. Four systems were probed: large cation + large anion, large cation + small anion, small cation + large anion, small cation + small anion. Our results show that the structuring of the ionic liquid is driven by the electrolyte-electrode interactions in the ultracapacitor, which are predominantly of the van der Waals type. Storage density energies are similar for all ultracapacitors, being in the range of 24 to 28 J cm-3 at 5.0V. Our results present a comparative analysis of the performances of four different ILs confined between two graphene electrodes. Although the best performance has been observed for the IL with ions (cations and anions) of equal sizes, no definite conclusion about the correlation of the performance to the ionic size ratio can be made from the present study.Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, São José dos Campos, SP, BrazilP.E.S., Vasilievsky Island, Saint Petersburg, Leningrad Oblast, Russian FederationDepartment of Physics, St. Petersburg State University, St. Petersburg, Russian FederationInstituto de Ciência e Tecnologia, Universidade Federal de São Paulo, São José dos Campos, SP, Brazi

Graphene-Based Systems for Energy Storage

Development of graphene-based energy storage devices based on the Laser Scribe system developed by the University of California Los Angeles. These devices These graphene-based devices store charge on graphene sheets and take advantage of the large accessible surface area of graphene (2,600 m2g) to increase the electrical energy that can be stored. The proposed devices should have the electrical storage capacity of thin-film-ion batteries but with much shorter charge discharge cycle times as well as longer lives The proposed devices will be carbon-based and so will not have the same issues with flammability or toxicity as the standard lithium-based storage cells

Vibrational spectroscopy at electrolyte/electrode interfaces with graphene gratings.

Microscopic understanding of physical and electrochemical processes at electrolyte/electrode interfaces is critical for applications ranging from batteries, fuel cells to electrocatalysis. However, probing such buried interfacial processes is experimentally challenging. Infrared spectroscopy is sensitive to molecule vibrational signatures, yet to approach the interface three stringent requirements have to be met: interface specificity, sub-monolayer molecular detection sensitivity, and electrochemically stable and infrared transparent electrodes. Here we show that transparent graphene gratings electrode provide an attractive platform for vibrational spectroscopy at the electrolyte/electrode interfaces: infrared diffraction from graphene gratings offers enhanced detection sensitivity and interface specificity. We demonstrate the vibrational spectroscopy of methylene group of adsorbed sub-monolayer cetrimonium bromide molecules and reveal a reversible field-induced electrochemical deposition of cetrimonium bromide on the electrode controlled by the bias voltage. Such vibrational spectroscopy with graphene gratings is promising for real time and in situ monitoring of different chemical species at the electrolyte/electrode interfaces