High-capacity hydrogen storage by metallized graphene

Abstract

First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li2s orbital is donated to graphene and is accommodated by its distorted π * -bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %.Developing safe and efficient hydrogen storage is essential for hydrogen economy.1 Recently, much effort has been devoted to engineer carbon based nanostructures 2,3,4,5 which can absorb H 2 molecules with high storage capacity, but can release them easily in the course of consumption in fuel cells. Insufficient storage capacity, slow kinetics, poor reversibility and high dehydrogenation temperatures have been the main difficulties towards acceptable media for hydrogen storage.Recently, graphene, a single atomic plane of graphite, has been produced 6 showing unusual electronic and magnetic properties. In this letter, we predict that metallized graphene can be a potential high-capacity hydrogen storage medium. The process is achieved in two steps: Initially, graphene is metallized through charge donation by adsorbed Li atoms to its π * -bands. Subsequently, each positively charged Li ion can absorb up to four H 2 by polarizing these molecules. At the end, the storage capacity up to the gravimetric density of g d =12.8 wt % is attained. These results are important not only because graphene is found to be a high capacity hydrogen storage medium, but also because of its metallization through Li coverage is predicted.Our results have been obtained by performing first-principles plane wave calculations using ultra-soft pseudopotentials.7 We used Local Density Approximation (LDA), since the van der Waals contribution to the Li-graphene interaction has been shown 8 to be better accounted by LDA. Numerical results have been obtained by using VASP, 9 which were confirmed by using the PWSCF code.10 A plane-wave basis set with kinetic energy cutoffh 2 |k + G| 2 /2m = 380 eV has been used. In the self-consistent potential and total energy calculations the Brillouin zone has been sampled by (19x19x1) and (9x9x1) special mesh points in k-space for (2×2) and (4×4) graphene cells, respectively. Atomic positions in all structures are optimized using the conjugate gradient method. Convergence is achieved when the difference of the total energies of last two consecutive steps is less than 10 −6 eV and the maximum force allowed on each atom is less than 10 −2 eV/Å. All configurations studied in this work have also been calculated by using spin-polarized LDA, which were resulted in non-magnetic ground state.Adsorption of a single (isolated) Li atom on the hollow site of graphene (i.e. H1-site above the center of hexagon) is modelled by using (4×4) cell of graphene w...

Tailoring Li adsorption on graphene

“…This optimal structure is found in the ( ) 2 3 2 3 30 R ×° supercell (Table 1), which contains 24 C and 4 Li atoms. All the Li atoms prefer to reside over hollow sites of the graphene sheet, consistent with previous results [20][21][22][23][24][25]. However, unlike uniform distribution pattern (which has symmetry group of P6mm); the four Li atoms form a rhombus structure.…”

“…To the best of our knowledge, most of the theoretical works thus far assumed, in analogy with multilayer graphite intercalated compounds (GIC), that the Li atoms are uniformly distributed on the graphene surface [24,[27][28][29][30]. However, due to the inactive delocalized π orbitals of graphene, the diffusion barrier of Li atoms on its surface is small.…”

“…Our choice of Li decorated graphene sheet [20][21][22][23] as a test case is motivated because of its potential in hydrogen storage [24], Li-ion battery [25,26], and superconducting materials [27][28][29]. To the best of our knowledge, most of the theoretical works thus far assumed, in analogy with multilayer graphite intercalated compounds (GIC), that the Li atoms are uniformly distributed on the graphene surface [24,[27][28][29][30].…”

Critical Insight into the Relentless Progression Toward Graphene and Graphene‐Containing Materials for Lithium‐Ion Battery Anodes

“…Singlelayer graphene behaved radically different instead and the intercalation stage "1", corresponding to the LiC 6 stoichio metry for the fully lithiated graphite, could not be reached. Based also on previous models developed few years before, [ 54,55 ] the authors proposed that the strong Coulombic repulsion of Li atoms facing opposite sides of the same graphene sheet results on lower binding energies, likely leading to a very low surface coverage equivalent only to a graphitic intercalation "stage 20" (i.e., stoichiometry of LiC 120 ). These fi ndings were further supported, few years later (i.e., in 2013), by a DFT study from B. I. Yakobson et al [ 56 ] showing that, in single-layer graphene, the formation of Li clusters is energetically favored with respect to any stable Li-graphene phase.…”

Environmental applications using graphene composites: water remediation and gas adsorption

“…165 Theoretical predictions have shown that in Li-decorated graphene, hydrogen uptake can be increased to 12 wt% due to the induced electric eld created by Li covered graphene. 166 In addition, a model calculation on welldesigned porous graphene decorated with Li is shown to be potentially capable of adsorbing hydrogen up to 12 wt%. 167 Besides the physisorption of hydrogen on carbon surfaces, the incorporation of hydrogen into framework structures is another promising method to store hydrogen gas.…”