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- ItemEnhanced electrochemical energy storage by nanoscopic decoration of endohedral and exohedral carbon with vanadium oxide via atomic layer deposition(Washington D.C. : American Chemical Society, 2016) Fleischmann, Simon; Jäckel, Nicolas; Zeiger, Marco; Krüner, Benjamin; Grobelsek, Ingrid; Formanek, Petr; Choudhury, Soumyadip; Weingarth, Daniel; Presser, VolkerAtomic layer deposition (ALD) is a facile process to decorate carbon surfaces with redox-active nanolayers. This is a particularly attractive route to obtain hybrid electrode materials for high performance electrochemical energy storage applications. Using activated carbon and carbon onions as representatives of substrate materials with large internal or external surface area, respectively, we have studied the enhanced energy storage capacity of vanadium oxide coatings. While the internal porosity of activated carbon readily becomes blocked by obstructing nanopores, carbon onions enable the continued deposition of vanadia within their large interparticle voids. Electrochemical benchmarking in lithium perchlorate in acetonitrile (1 M LiClO4) showed a maximum capacity of 122 mAh/g when using vanadia coated activated carbon and 129 mAh/g for vanadia coated carbon onions. There is an optimum amount of vanadia between 50 and 65 wt % for both substrates that results in an ideal balance between redox-activity and electrical conductivity of the hybrid electrode. Assembling asymmetric (charge balanced) full-cells, a maximum specific energy of 38 Wh/kg and 29 Wh/kg was found for carbon onions and activated carbon, respectively. The stability of both systems is promising, with a capacity retention of ∼85–91% after 7000 cycles for full-cell measurements.
- ItemNew insights into the structure of nanoporous carbons from NMR, Raman, and pair distribution function analysis(Washington D.C. : American Chemical Society, 2015) Forse, Alexander C.; Merlet, Céline; Allan, Phoebe K.; Humphreys, Elizabeth K.; Griffin, John M.; Aslan, Mesut; Zeiger, Marco; Presser, Volker; Gogotsi, Yury; Grey, Clare P.The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons.
- ItemRing current effects: Factors affecting the NMR chemical shift of molecules adsorbed on porous carbons(Washington D.C. : American Chemical Society, 2014) Forse, Alexander C.; Griffin, John M.; Presser, Volker; Gogotsi, Yury; Grey, Clare P.Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures.
- ItemSub-micron novolac-derived carbon beads for high performance supercapacitors and redox electrolyte energy storage(Washington D.C. : American Chemical Society, 2016) Krüner, Benjamin; Lee, Juhan; Jäckel, Nicolas; Tolosa, Aura; Presser, VolkerCarbon beads with sub-micrometer diameter were produced with a self-emulsifying novolac–ethanol–water system. A physical activation with CO2 was carried out to create a high microporosity with a specific surface area varying from 771 (DFT) to 2237 m2/g (DFT) and a total pore volume from 0.28 to 1.71 cm3/g. The carbon particles conserve their spherical shape after the thermal treatments. The controllable porosity of the carbon spheres is attractive for the application in electrochemical double layer capacitors. The electrochemical characterization was carried out in aqueous 1 M Na2SO4 (127 F/g) and organic 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (123 F/g). Furthermore, an aqueous redox electrolyte (6 M KI) was tested with the highly porous carbon and a specific energy of 33 W·h/kg (equivalent to 493 F/g) was obtained. In addition to a high specific capacitance, the carbon beads also provide an excellent rate performance at high current and potential in all tested electrolytes, which leads to a high specific power (>11 kW/kg) with an electrode thickness of ca. 200 μm.