Few-layer graphene as an ‘active’ 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 ‘active’ conductive additive in flexible activated carbon-based aqueous electric double layer capacitor (supercapacitor) electrodes if introduced by a novel ‘vacuum infiltration’ 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 g−1 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 g−1 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 (3 ± 1 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 “roughened” 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 theory–based 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 so‐called ‘impulse approximation’, 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