43 research outputs found
Few-layer graphene as an âactiveâ conductive additive for flexible aqueous supercapacitor electrodes
We demonstrate that few layer graphene (FLG), formed by high-shear exfoliation into an aqueous suspension, can be successfully employed as an âactiveâ conductive additive in flexible activated carbon-based aqueous electric double layer capacitor (supercapacitor) electrodes if introduced by a novel âvacuum infiltrationâ technique. The effectiveness of the FLG can be optimised by tailoring its size distribution and loading. It is found that best performance is achieved using FLG with the broadest size distribution and, moreover, that the larger size distribution is effective over the broadest range of loading. With optimum size distribution and loading, FLG is shown to outperform a commercial carbon black conductive additive (Timcal C65). Electrodes containing 8 wt% infiltrated FLG have an equivalent series resistance (ESR) of 1 . 3 ± 0 . 4 Ω , and a specific capacitance of 142 . 3 ± 0 . 1 F gâ1 over a voltage window of 1.2 V, compared with an ESR of 3 ± 1 Ω and a specific capacitance of 96 . 81 ± 0 . 02 F gâ1 for equivalent electrodes produced with an optimal loading of carbon black additive. As a result, the specific energy density of electric double layer capacitors (EDLCs) produced with a vacuum infused FLG additive is demonstrated to be an average of 47 ± 3 % superior to those containing carbon black measured at similar power densities. In contrast to vacuum infusion, direct mixing of FLG suspension into the electrodes is found to be ineffective, resulting in limited improvement relative to electrodes without conductive additive, the reasons for which are discussed
Facile technique for the removal of metal contamination from graphene
Metal contamination deposited on few-layer graphene (3â±â1 monolayers) grown on SiC(0001) was successfully removed from the surface, using low cost adhesive tape. More than 99% of deposited silver contamination was removed from the surface via peeling, causing minimal damage to the graphene. A small change in the adhesion of graphene to the SiC(0001) substrate was indicated by changes observed in pleat defects on the surface; however, atomic resolution images show the graphene lattice remains pristine. Thin layers of contamination deposited via an electron gun during Auger electron spectroscopy/low energy electron diffraction measurements were also found to be removable by this technique. This contamination showed similarities to âroughenedâ graphene previously reported in the literature
Proof for trivalent Sc ions in Sc2@C84 from high-energy spectroscopy
The electronic structure and the valency of the Sc ions in the endohedral dimetallofullerene Sc 2 @C 84 with
D 2d symmetry are probed using high-energy spectroscopy. Comparison of the Sc 2p ! 3d x-ray-absorption
spectrum with calculated ionic multiplet spectra shows that the Sc ions are trivalent. Detailed multiplet calcu-lations
including covalency indicate that the effective valency of the Sc~III! ions can be described by a formal
charge transfer to the fullerene cage of 2.660.1. This illustrates that a purely ionic picture is not valid for the
electronic structure of Sc 2 @C 84 , and that a more complex picture including finite hybridization between the Sc
and the fullerene cage has to be applied
Sodium atoms and clusters on graphite: a density functional study
Sodium atoms and clusters (N<5) on graphite (0001) are studied using density
functional theory, pseudopotentials and periodic boundary conditions. A single
Na atom is observed to bind at a hollow site 2.45 A above the surface with an
adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates
a flat potential energy surface. Increased Na coverage results in a weak
adsorbate-substrate interaction, which is evident in the larger separation from
the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The
binding is weak for Na_2, which has a full valence electron shell. The presence
of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and
both Na_4 and Na_5 are distorted from planarity. The calculated formation
energies suggest that clustering of atoms is energetically favorable, and that
the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite
than in the gas phase. Analysis of the lateral charge density distributions of
Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure
Adsorption of C_6_0 on surfaces
SIGLEAvailable from British Library Document Supply Centre-DSC:D197447 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Extraordinarily Long-Ranged Structural Relaxation in Defective Achiral Carbon Nanotubes
We present a systematic ab initio density functional theoryâbased study which demonstrates that even one of the simplest defects in single-wall carbon nanotubes, the reconstructed monovacancy (a pentagonal ring and a single dangling bond known as a 5-1db defect), leads to extraordinarily long-ranged structural distortions. We show that relaxation due to reconstruction can only be modeled accurately through a careful selection of boundary conditions and an appropriately long nanotube fragment
Electron Compton scattering and the measurement of electron momentum distributions in solids
Electron Compton scattering is a technique that gives information on the electron momentum density of states and is used to characterize the ground state electronic structure in solids. Extracting the momentum density of states requires us to assume the soâcalled âimpulse approximationâ, which is valid for large energy losses. Here, the robustness of the impulse approximation in the low energy transfer regime is tested and confirmed on amorphous carbon films. Compared to traditional Compton measurements, this provides additional benefits of more efficient data collection and a simplified way to probe valence electrons, which govern solid state bonding. However, a potential complication is the increased background from the plasmon signal. To overcome this, a novel plasmon background subtraction routine is proposed for samples that are resistant to beam damage