Tsuyoshi Sugimoto

Electrolyte roadblocks to a magnesium rechargeable battery

Low cost, non-dendritic magnesium metal is an ideal anode for a post lithium ion battery. Currently, development of magnesium electrolytes governs the rate of progress in this field, because electrolyte properties determine the class of cathodes utilized. A review of the latest progress in the area of magnesium battery electrolyte and a perspective on mitigating present challenges is presented herein. Firstly, density functional theory has been shown to predict the potential window of magnesium electrolytes on inert electrodes. Secondly, we report initial efforts aimed to overcome the corrosive property of these magnesium organohaloaluminates towards less noble metals such as stainless steel. This is a major challenge in developing high voltage magnesium electrolytes essential for batteries which operate above 3V. We lastly touch on cathode candidates including the insertion and conversion classes. One conversion cathode we pay particular attention to is electrophilic sulfur which can be married with magnesium metal anodes by utilizing non-nucleophilic electrolytes obtained by simple crystallization of in situ generated magnesium organohaloaluminates. Effectively, non-nucleophilic electrolytes open the door to research on magnesium/sulfur batteries.

First principles studies for the dissociative adsorption of H2 on graphene

We investigate and discuss the interaction of H2 with graphene based on density functional (DFT) theory. We calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites on graphene. Our calculation results show that reconstructions of the carbon atoms play an important role in the H2 -graphene interactions. Activation barrier for H2 dissociation on an unrelaxed graphene is considerably high, ∼4.3 eV for a T–H–T geometry and ∼4.7 eV for a T–B–T geometry. The T–H–T(T–B–T) geometry means that the center of mass position of H2 is at the hollow(bridge) site, and the two H atoms are directed towards the top sites on the graphene. On the other hand, when the carbon atoms are allowed to relax, the activation barrier decreases, and becoming 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry. In this case, the two carbon atoms near the hydrogen atoms move 0.33 A towards the gas phase for the T–H–T geometry and 0.26 A for the T–B–T geometry.

Effective Pathway for Hydrogen Atom Adsorption on Graphene

We investigate and discuss the interaction of a hydrogen atom (H) with graphene based on the density functional theory (DFT). Our calculation results show that reconstructions of carbon atoms play an important role in the H adsorption on graphene. When constituent carbon atoms are held rigid, endothermic H adsorption is about 0.2 eV, and the activation barrier is 0.3 eV for H adsorption, due to the strong π-bonding network of the hexagonal carbon. On the other hand, when carbon atoms are allowed to relax, the carbon atom directly below the H atom moves 0.33 A upward towards the gas phase, and an s p 3 -like geometry is formed between the H and carbon atoms of graphene. This relaxation stabilizes the hydrogen–carbon interaction, and the exothermic hydrogen adsorption on the graphene has a binding energy of 0.67 eV. We also show that the effective pathway for H adsorption on graphene, which gives an activation barrier for the H adsorption on graphene of 0.18 eV.

Realizing a carbon-based hydrogen storage material

In response to the current need for an efficient, safe, and compact system for storing hydrogen in mobile applications, a scheme for maximizing and controlling hydrogen storage in graphite is proposed by modifying substrate reactivity through the exploitation of intrinsic vibrational modes in pristine and fully-hydrogenated graphite systems. Calculations within density functional theory suggest that infrared radiation of distinct frequencies can be used to independently induce graphite lattice restructuring and recrystallization for promoting hydrogen uptake and discharge, respectively. Effects of the initial attachment of hydrogen on graphite sheets are discussed, with computational results showing that additional hydrogen adsorption can proceed through easier reaction routes.

Scattering and dissociative adsorption of H2 on the armchair and zigzag edges of graphite

We performed quantum dynamics calculations on the scattering and dissociative adsorption of hydrogen molecules incident on the armchair and zigzag edges of graphite layers, using relevant potential-energy surfaces (PESs) recently obtained by Dino et al. [e-J. Surf. Sci. Nanotech. 2, 77 (2003), and references therein]. By employing the coupled channel method to determine the reflection and sticking probabilities, we compared the hydrogen scattering and dissociative adsorption dynamics on the two graphite surfaces. Our findings show the different scattering behaviors of H2 for the armchair edge and for the zigzag edge, which enable the identification of an unknown graphite edge from its interaction with H2. The scattering on the zigzag edge is due to the highly curved region of the PES reaction path for H2 interacting with the zigzag edge, whereas the scattering for the armchair edge is caused by a potential barrier. The reflection probability initially decreases with increasing the kinetic energy in both c...

Stability of Three-Hydrogen Clusters on Graphene

Realizing high hydrogen uptakes on surfaces is one of the essential aspects of practical hydrogen storage in solid-state materials. To achieve this, it is always beneficial to know how the road to adsorption saturation on the surface looks like in terms of the physical mechanisms involved, and how we can control required reactions given this knowledge. On this topic, work has been carried out on how the simplest groups—pairs—of hydrogen behave on graphite/graphene. In that study it was shown that hydrogen pair interaction cannot be described by a simple function of interadsorbate separation, and that only certain pairing geometries on the surface are energetically favored (we note here that full relaxation of the substrate atoms was not found to change these conclusions). As detecting and discriminating singly adsorbed and small groups of adsorbates is an essential step to knowing how saturation can be reached, we have subsequently shown how probing surface electronic states can be used to identify an atomic hydrogen adsorbate, and distinguish it from the closely-spaced hydrogen pairs on the surface, affirming previously published experimental work on this subject, particularly that in ref. 3. In this paper we comment on the next step towards saturation: the formation of hydrogen clusters of three, i.e. hydrogen trios, on graphene. Results are discussed with respect to results obtained from hydrogen pairs adsorbed on graphene. Stable hydrogen adsorption configurations were determined through geometry optimization calculations using the VASP code, which implements the projector augmentedwave method for density functional theory-based electronic structure calculations. All calculations were spin-polarized, and utilized the exchange–correlation functional based on the PBE version of the generalized gradient approximation. We applied a 400 eV cutoff to limit the plane-wave basis set without compromising computational accuracy, and a 4 4 1 Monkhorst–Pack special k point grid for Brillouin zone sampling. Three H atoms on a 48-C atom single sheet comprise the unit cell, with C–C nearest-neighbor distances of 1.42 A before relaxation. All atoms were completely unrestricted in the geometry optimization. A 15.0 A vacuum separating adjacent sheets was used. Figure 1 shows the different clusters of three hydrogen atoms systems included in the computations of this study. We specifically choose the fourteen most closely-packed combinations of three atoms adsorbed on C atom ‘‘top’’ sites. Upon reconstruction hydrogen atom lateral positions generally don’t deviate much from the positions on receiving C atoms shown in Fig. 1. A trio is named based on its smallest H pairing component (o = ortho, m = meta, p = para) and distance of the third member of the trio from the pair center. This means, for example, that the trio labeled to1 is the three-hydrogen cluster comprised of a pair of adjacently adsorbed (ortho) hydrogen, and a third H atom adsorbed in the closest possible distance from the aforementioned pair. Table I shows the trios arranged by adsorption energy, starting with the most stable geometry. Table values were computed using the following expressions: Eads 1⁄4 Egr+3H(ads) ðEgr þ 3EH(g)Þ; Es 1⁄4 Eads=3; Eo 1⁄4 Egr+3H(ads) ðEgr+2H(ortho) þ EH(g)Þ; Em 1⁄4 Egr+3H(ads) ðEgr+2H(meta) þ EH(g)Þ; and Ep 1⁄4 Egr+3H(ads) ðEgr+2H(para) þ EH(g)Þ, where the terms Egr, EH(g), Egr+3H(ads), and Egr+2H are the total energies for the graphene sheet, a gas phase H atom, the system comprised of an adsorbed H trio and graphene, and the system comprised of an adsorbed H pair and graphene, respectively. The adsorption energy of an isolated H atom on graphene Eiso 1⁄4 Egr+H(ads) ðEgr þ EH(g)Þ is 0:77 eV, a value which differs slightly from previous calculations due to the different adsorbate coverage used in these studies. For the same reason pair and trio adsorption energies reported here also differ slightly from corresponding values reported in refs. 2 and 8. Adsorbed trios are generally stable: the Eads (and Es) values are all negative, meaning all adsorbed three-hydrogen groups are stable with respect to hydrogen atoms located far from the graphene surface. If Es for a given trio is less than Eiso, the trio is more stable compared with a system comprised of three isolated adsorbed H on graphene. In other words it would be more energetically favorable for the hydrogen atoms to clump together than to separate from each other on the surface, i.e., the net interaction shows an ‘attractive’ character. Only one trio— tm1— is found to1 to4

Stable hydrogen configurations between graphite layers

Based on the density functional theory, we investigate and discuss what possible stable configurations hydrogen might assume, once found inside/between graphite layers stacked in an A B configurati...

First Principles Studies on the Interaction of a Hydrogen Atom with a Single-Walled Carbon Nanotube

We investigate the interaction of a hydrogen atom with an armchair-type, single-walled carbon nanotube, based on the density functional theory (DFT), and discuss how changes in the nanotubes diameter affect hydrogen adsorption on the carbon nanotube. We find that because the strong sp2-like bonding and π-bonding network are enhanced with increasing nanotube diameter, hydrogen adsorption on the carbon nanotube becomes less stable with increasing nanotube diameter. We also find that with increasing nanotube diameter, the potential energy curves approach that for hydrogen interacting with a graphene sheet. These results can be explained in terms of the hydrogen-induced changes in the local density of states of the carbon nanotube.

Identifying Hydrogen Atoms on Graphite

We comment on the identification of a hydrogen atom adsorbed on graphite and distinguishing it from closely-spaced pairs under the scanning tunneling microscope (STM) through electronic state calculations based on density functional theory. The presence of the H atom should be very well observable through a distinct feature most directly associated with the adsorbate itself, a threefold symmetry most apparent through the third-nearest neighbor C atoms, and through sublattice visibility differences. A comparison with effects on the electronic states brought about by closely-spaced pairs shows visible differences in the aforementioned three factors, which should enable us to discriminate among adsorbed structures. Results compare well with STM measurements of adsorbed deuterium on graphite.