First-Principles Calculations Show Ultrafast Electron Injection for Dye Adsorbed on TiO 2 Nanowire
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Chemically Modified Graphene Stars as New Electrode Material in Ultracapacitor Cell Capacitors have numerous technological applications, including energy storage, and researchers are striving to develop more efficient devices (i.e., to increase capacitance per unit mass or per unit volume or both). Some ultracapacitors utilize nanoscopic charge separation at the electrochemical interface between an electrode and an electrolyte, an approach known as electrochemical double layer capacitance (EDLC). They offer superior performance compared to conventional dielectric capacitors because the energy density is inversely proportional to the thickness of the double layer, and this thickness in EDLC is of the order of a nanometer. In the October issue of Nano Letters (DOI: 10.1021/nl802558y; p. 3498), M.D. Stoller, S. Park, Y. Zhu, J. An, and R.S. Ruoff of The University of Texas, Austin have introduced a class of carbon material for use as an electrode material in EDLC ultracapacitors, which they call chemically modified graphene (CMG). The researchers synthesize the CMG by suspending graphene oxide sheets in water, and then reduce the sheets with hydrazine hydrate. During reduction, the graphene sheets agglomerate into 15–25-μm-diameter particles that possess a surface area of 705 m 2/g as measured by the N2 absorption Brunauer– Emmett–Teller (BET) method, and the oxygen content is greatly reduced by the chemical action of hydrazine, so that the “reduced graphene oxide” sheets have excellent electrical conductivity. The impressively high surface area supports CMG’s promise as a useful material for ultracapacitor electrodes. The researchers have also built a twoelectrode EDLC cell. They form CMG particles into porous electrodes using a polytetrafluoroethylene binder. The cell consists of two such electrodes, isolated by a porous separator, with all of this saturated by an electrolyte that permits ionic current while suppressing electronic current that would otherwise discharge the cell. The high conductivity of the CMG materials permits thicker electrodes that also are free of the conductive fillers that had been previously required. This raises the relative amount of electrode material, and boosts the energy density of the packaged ultracapacitor. The researchers used cyclic voltammetry and galvanostatic charge/discharge to measure the specific capacitance of the cells. Values between 80–135 F/g are obtained, using three electrolytes commonly found in commercial EDLCs. They also used electrical impedance spectroscopy to
characterize the frequency response and hence the equivalent series resistance (ESR) of the cell. The high conductivity of the CMG materials contributes to the favorable values of ESR and power capability. “These chemically modified graphenes are cheap and abundantly available, have good electrical conductivity, and very large surface areas,” said Stoller and Ruoff. “Furthermore, they are compatible with common electrolyte systems. Ultracapacitors based on these materials could have the cost and performanc
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