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Storing-hydrogen processes on graphene activated by atomic-vacancies
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Storing-hydrogen processes on graphene activated by atomic-vacancies Gagus Ketut Sunnardianto a,*, Isao Maruyama b, Koichi Kusakabe a a
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-chou, Toyonaka, Osaka, 560-8531, Japan b Faculty of Information Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro, Higashi, Higashi-ku, Fukuoka, 811-0295, Japan
article info
abstract
Article history:
We investigated the minimum energy pathways and energy barriers of reversible reaction
Received 10 November 2016
(V111 þ H24V221) based upon calculations using density functional theory. We find a
Received in revised form
comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and
11 January 2017
desorption processes. The charge transfer rate from a reacting hydrogen atom to the
Accepted 19 January 2017
graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path
Available online xxx
to recover the initial structure of V111 is realized by the migration of hydrogen atoms from
Keywords:
tion and desorption suggests that this novel storage and release concept has the potential
Graphene vacancy
to act as a hydrogen storage system for certain applications.
V221 onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorp-
Adsorptionedesorption
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Activation barrier
Introduction Nowadays, with the aim of realizing a hydrogen society, much effort is being been directed toward finding solutions to several unresolved problems in hydrogen-based technologies [1e6]. As a relevant subject with the potential to reduce financial costs, innovation is necessary in hydrogen storage for long-distance transport. Organic hydrides (OHs) including methyl-cyclohexane have been considered as a next-stage solution that is now under development [7e11]. However, a problem with OHs is that they are difficult to dehydrogenate, which demands a properly chosen catalyst [12]. Since the discovery of graphene and its remarkable properties [13e15], this material has attracted much attention and is providing promising candidates for carbon-based materials in various fields related to material science. This is
particularly so for the problem of hydrogen storage, for which graphene is regarded as particularly promising [16e27] because of its hydrogenationedehydrogenation reactions. Hydrogenated graphene was first realized by Elias et al. [18], who demonstrated that the hydrogenation reaction is reversible. This characteristic makes hydrogenated graphene a potential candidate for hydrogen storage [20]. Thus, the interaction between graphene and hydrogen is now the subject of much discussion [28e32]. However, there remain some barriers to bring into real applications. It is well known that a hydrogen molecule interacts weakly with pristine graphene. In molecular hydrogen adsorption, it is physisorption that occurs primarily, and the dissociation rate of hydrogen molecules on pristine graphene is very low. This is because the energy barriers against dissociating a H2 molecule on pristine graphene are high, amounting to as much as 2.7e3.3 eV [33,34]. The chemical inertness of a graphene
* Corresponding author. E-mail address: [email protected] (G.K. Sunnardianto). http://dx.doi.org/10.1016/j.ijhydene.2017.01.115 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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surface against molecular hydrogen limits the volume density of hydrogen in the molecular adsorption form [24]. Starting from atomic hydrogen, however, the resultant adsorbed surface is in another form with chemisorbed hydrogen atoms. Atomic hydrogen reacts with the graphene more easily over a wide range of coverage [35]. However, in general, easy adsorption leads to difficult desorption; the desorption energy barrier amounts to 1.4 eV [36]. Thus, although we have sufficient hydrogen density for storage use when chemically hydrogenated graphene is created, we have to solve the problem of the desorption barrier. Previous study, we have succeeded in deriving the rule that determines the charge transfer rate (CTR) from hydrogen to graphene over a wide range of coverage. Our findings reveal that the CTR shows roughly linear behavior from 0.22e for the dilute limit to 0.15e for the half-coverage limit [37]. The structure that appears in the half-coverage limit is called graphone [38]. Thus, different desorption characteristics may be expected when the coverage is controlled. Although desorption would be easier for a higher coverage, we should search for an enhancement effect that would work for every coverage. Accordingly, we seek a suitable method for enhancing the desorption rate from a properly prepared hydrogenated graphene surface. The demand for enhanced reactivity has motivated researchers to modify graphene chemically. A plausible way to increase the interaction strength is by forming a vacancy in the graphene. Recently, vacancy defects in graphene have attracted considerable attention in the field of material science [14,39e49]. Atomic vacancies can be created by irradiating graphene with Arþ ions with typical ion energy of 100 eV and exposure times of 3e4 s [50,51]. Furthermore, hydrogenation treatment of graphene with atomic vacancies creates various types of hydrogenated atomic vacancy [50]. Single-vacancy graphene hydrogenated with molecular hydrogen has been realized experimentally [52]. An effect of the reaction has been observed through the apparent change in the conduction properties with respect to the bias gate voltage [52]. This is evidence for the reactivity of hydrogen molecules at the vacancy sites. In fact, successive reactions of molecules at a vacancy may be realized [53,54]. The authors (GKS, I.M and K.K) reported the plausibility of hydrogen-dissociative adsorption on a graphene monovacancy to form atomic on-top hydrogen at an in-plane graphene surface by migration of a hydrogen atom [54]. After initial hydrogenation of an atomic vacancy to form a hydrogenated graphene monovacancy V11 (a graphene vacancy with two hydrogen atoms at the vacancy edge), dissociative chemisorption of H2 on V11 to form V211 (a hydrogenated graphene vacancy at which two carbon atoms are monohydrogenated and one carbon atom is di-hydrogenated at the vacancy edge) is possible. The activation barrier for dissociative chemisorption of a H2 molecule on graphene mono-vacancy V11 is around 0.5 eV. The diffusion reaction of one hydrogen atom on V211 then makes the hydrogen atom migrate on the graphene surface to form V111 þ H. Here the second barrier to migration may be compensated for by the 2.5-eV reaction energy of the first adsorption reaction [54]. To achieve efficient hydrogen storage, fundamental research such as i) the adsorption and desorption
characteristics, ii) the nature of hydrogen diffusion or migration, and iii) the activation barrier of the adsorption or desorption direction is required by specifying the reaction paths to be investigated. In such a path, the initial and final states have to be identified. The discovery of a relevant path may then lead us to have a concept of the self-catalytic reaction that is selected from all possible paths. The resulting path would answer the question of whether hydrogen uptake occurs endothermically or exothermically, or indeed whether energy release takes place or not during hydrogen desorption. In this newly planned study, we report the comparable values of activation barriers for both adsorption and desorption of one hydrogen molecule on a triply-hydrogenated vacancy V111 of graphene. We consider the characteristic reaction path of V111 þ H2 forming V221, in the latter of which two carbon atoms are di-hydrogenated and one carbon atom is mono-hydrogenated at the vacancy edge. This process is reversible. Interestingly, the adsorption and desorption barriers are comparable at around 1.3 eV. The reaction energy is almost negligible, which is useful for hydrogen storage applications. Our discovery of the reversible reaction V111 þ H2 4 V221 leads us to a novel concept of the self-catalytic property of hydrogenated graphene vacancies. The reaction barrier is reduced by the creation of hydrogenated vacancies, typically as a form of V111. The 1.3-eV energy barrier is valuable for applications because it allows us to protect against unfavorable release of hydrogen at low temperatures and near ambient conditions. Once the system is heated above a few hundred Kelvin, hydrogen may be released from V221. In the summary and discussion part of this paper, we discuss plausible evidence from existing experiments for the lift-off process [52]. Our findings may be used as a foundation and guidance for the innovative design of defective graphene as a material for the optimum storage and release of hydrogen.
Materials and methods The structure models used for the simulation were graphene sheets in a rectangular supercell that consists of 63 carbon atoms. Calculations have been done using density functional theory (DFT) [55] implemented in the QUANTUM ESPRESSO code [56]. Local density approximation by the Perdew-Zunger parametrization was adopted [57,58]. We used the projectoraugmented wave method and an ultra-soft pseudo potential [59,60]. All atomic visualization were created using XCrySDen [61]. The nudged-elastic-band method (NEB) was applied to determine the minimum energy pathway of a reaction path and the energy barrier for both the dissociative-chemisorption and desorption processes [62,63]. The parameters were an energy cut-off of 40 Ry for the plane-wave expansion of the wave function and 400 Ry for the expansion of the augmented charge. The convergence criterion for the structural optimization was that the total absolute value of the inter-atomic force vector should be less than A 104 Ry/a.u. The distance between graphene planes was 10 in order to avoid interaction between layers. A distribution of k-points on a 16 16 1 mesh was used via the MonkhorstePack scheme [64]. Along the reaction pathways, the
Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
€ wdin charge analhydrogen charge was obtained by using Lo ysis [65]. Because hydrogen atoms repel each other geometrically at a vacancy, the hydrogenated vacancy is not planar. We identified a buckled hydrogenated carbon network, the detailed structure of which depends on the orientation of the adsorbed hydrogen atoms. If a hydrogen atom is adsorbed at the top of a carbon atom, the hydrogenated carbon atom buckles out of the plane because of the formation of local sp3-like bondings. To check whether the chosen calculation parameters were sufficiently accurate, we performed structural optimization of single hydrogen adsorption on graphene as well as the V111 structures. The benchmark results showed that the CeH bond length was 1.12 A and that the buckling of graphene was from the shift of an adsorbed carbon atom upward by 0.38 A from the graphene plane. These numerical values are almost the same as those found in available references [17,30,33,35,66e72]. For the V111, the buckling was estimated to be due to an upward shift of hydrogenated carbon of 0.19 A, which again is in reasonable agreement with previous results [42,53].
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Fig. 1 e Minimum energy pathway from V111 þ H2 to V221. The blue and red atoms denote carbon and hydrogen, respectively. Symbols IS, TS, FS, u, d, and sd represent the initial state, transition state, final state, up configuration, down configuration, and slightly down configuration, respectively.
Results and discussion Reaction pathways of H2 dissociative adsorption on the V111 surface We previously reported the reaction of a hydrogen molecule at a V11 surface to form V211 [54]. Here V11 is a graphene vacancy with two hydrogen atoms at the vacancy edge, whereas V211 is a graphene vacancy for which two carbon atoms are monohydrogenated at the vacancy edge and one is dihydrogenated. The energy barrier for this reaction is around 0.5 eV [54]. Moreover, the V211 structure has a reaction path to the V111 with a migrated hydrogen atom on the in-plane graphene [54]. Once the migration reactions occur, the local V111 structure can allow further hydrogenation to form a V221 structure in the molecular hydrogen treatment. Once V221 has formed at a certain hydrogen pressure, we expect a reversible reaction path between V111 þ H2 and V221. Here, we investigate the pathways for H2 dissociation on the V111 surface and its energy barrier (see Fig. 1). Firstly, in the initial state, the adsorbed H2 molecule is located on the hollow site of the vacancy. As the reaction proceeds, the hydrogen molecule (with its bond length of around 0.77 A) rotates a little, causing the energy of the system to increase. The bond elongates to 0.8 A, followed by dissociation of the hydrogen molecule with an energy barrier of around 1.28 eV. In the transition state (TS) as shown in Fig. 1, the H2 molecule dissociates into two H atoms weakly bound to the C atoms at the vacancy edge. After that, each hydrogen atom is adsorbed by two mono-hydrogenated carbon sites, forming V221 (u-ud-ud). Along the pathways, the configuration of V111 (u-d-sd) is changed prior to the dissociation of the hydrogen molecule to be V111 (u-d-d) at the transition state (Figs. 1 and 2). We observed that the energy difference between V111 þ H2 and V221 was relatively small at around 0.03 eV. The energy
barrier for the dissociative chemisorption at the graphene V111 surface was around 1.28 eV. In this case, a single vacancy can be regarded as a “self-catalytic” reaction center. This plays an important role in reducing the energy barrier. The dehydrogenation of V221 is the reverse reaction path of V111 þ H2 / V221, which gives a reaction path to form V111 þ H2 from V221. As the dehydrogenation proceeds, the hydrogen atom in the up orientation in the di-hydrogenated carbon site starts to desorb. Then, in the transition state, two of the desorbed hydrogen atoms start to form a H2 molecule in the vacuum, forming the final states V111 þ H2. It is interesting to note that the reversible reaction of V111 þ H2 4 V221 has comparable energy-barrier values for both adsorption and desorption. The small energy difference between the initial (V221) and final (V111 þ H2) states, this suggests efficient hydrogen release and uptake processes, making the storage device economically feasible. Thus, we reason that the pathways from V111 þ H2 / V221 and its reverse reaction represent good process designs for hydrogen storage. This is also due to the comparable activation-barrier values for both adsorption and desorption, which means that these reversible reactions are more thermodynamically plausible than the other competing materials, namely OHs.
Charge-transfer rate Information on the charge state of hydrogen could lead to a method for controlling the reversible reaction. In order to determine the change in the hydrogen charge along the pathways and the charge transfer rates from hydrogen to graphene, we performed charge analysis to calculate the values of hydrogen charge listed in Table 1.
Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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Fig. 2 e Structures appearing along the pathway. Left: initial state (IS); center: transition state (TS); right: final structure (FS) of V 221(u-ud-ud). The blue and red atoms denote carbon and hydrogen, respectively. In the transition state, one hydrogen atoms (number one) have changed from sd configuration in the initial state into d configuration in the transition state. Each hydrogen atom is numbered to identify the value of hydrogen charge shown in Table 1. Symbols u, d, and sd represent up, down, and slightly down configurations, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 e Values of hydrogen charge in the initial state V111 þ H2 (IS), transition state (TS), and final state V221 (uud-ud) (FS) of the pathway. The different H atoms are numbered as in Fig. 2. No. 1 2 3 4 5
IS charge [e]
TS charge [e]
FS charge [e]
0.82 0.80 0.81 0.99 0.99
0.81 0.81 0.83 0.89 0.91
0.80 0.80 0.83 0.82 0.82
We found that the charge of a neutral hydrogen molecule in the initial configuration has an almost symmetric distribution with respect to the molecular center, although weak interaction from the hydrogenated vacancy structure inclines the molecular axis toward the surface (see Fig. 3). The partial summation value of the charge is 0.99 e per hydrogen atom. In the transition state, the hydrogen molecule dissociates into two hydrogen atoms, which are numbered 4 and 5 in Fig. 2. The electron charge of hydrogen atom no. 4 decreases to 0.89 e, whereas that of hydrogen atom no. 5 decreases only to 0.91 e. In the final state, the charges of hydrogen atoms nos. 4 and 5 both decrease to 0.82 e. Thus, there is electron transfer from the hydrogen to the graphene mono-vacancy along the reaction path proceeding from the transition state to the final
state. In the final state, we find that the charge transfer from hydrogen to the graphene surface is around 0.18e per hydrogen. In the initial state, the inter-atomic distance d of the reacting atoms of the hydrogen molecule is around 0.77 A, whereas in the transition states an elongation of d ¼ 0.80 A occurs before dissociation of the hydrogen molecule.
Migration of a hydrogen atom from V221 It is possible to have several migration paths of atomic hydrogen on the defective graphene V221. We examined the barrier for diffusion of hydrogen atoms from the vacancy site to the graphene surface, i.e., V221 / V211 þ H/Gr. By looking at this process, we observed hydrogenation both at the vacancy and on the graphene surface. Once V221 was created, there were possible transition paths toward V211 with a migrated hydrogen atom on the graphene surface (H/Gr). In fact, V211 with migrated hydrogen may arise from V221. We observed an activation barrier for the migration of a hydrogen atom from V221 to V211 þ H/Gr. Here, the last hydrogen in V211 þ H/Gr was on top graphene surface, for which the total energy was approximately 0.8 eV higher than that for V221. Firstly, a hydrogen atom in one of the di-hydrogenated carbon sites migrates to the exact on-top site of the nearest bare carbon atom. After that, the hydrogen atom migrates to the next nearest carbon atom in the graphene plane.
Fig. 3 e Top (upper panel) and side (lower panel) views of charge density profiles appearing along the pathway. The electronic density increases from blue to white to red. Blue indicates electron deficit whereas red indicates electron excess. These figures were obtained using the XCrySDen graphical user interface [61]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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Fig. 4 e Minimum energy pathway from V221 to V211 þ H/ Gr, where H/Gr is an adsorbed hydrogen atom on the graphene plane. The blue and red atoms are carbon and hydrogen, respectively. Their atomic structures are given by the insets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This is a two-step reaction path. In the first step, one of the hydrogen atoms in the di-hydrogenated carbon site migrates to the nearest-neighbor bare carbon atom with an energy barrier of 1.57 eV. Subsequently, the migrated hydrogen atom diffuses to the second carbon atom in the graphene surface with an energy barrier of 1.67 eV (see Fig. 4). The energy of the product is higher than that of the reactant, so the migration process from V221 to V211þ (an adsorbed H on the graphene) is an endothermic reaction. Thus, there are two energy barriers for this two-step reaction: the first step requires an energy of 1.57 eV to cross the potential barrier, and the second step requires an energy of 1.67 eV to reach the final state. Once V211 þ H/Gr is realized, there is the possibility of further migration from V211 þ H/Gr to V111 þ 2H/Gr, where 2H/
Fig. 5 e Minimum energy pathway from V211 þ H/Gr to V111 þ 2H/Gr, where H/Gr is an adsorbed hydrogen atom on the graphene plane. Here, 2H/Gr denotes two adsorbed hydrogen atoms on the graphene plane. The blue and red atoms are carbon and hydrogen, respectively. Their atomic structures are given by the insets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Gr denotes two adsorbed hydrogen atoms on the graphene plane. Hence, we observed the migration pathways and the activation barrier of the migration from V211 þ H/Gr to V111 þ 2H/Gr. As shown in Fig. 5, there are again two transition states, both of which must be overcome to reach the final states. Firstly, a hydrogen atom migrates from the dihydrogenated carbon site to the nearest-neighbor bare carbon atom with an activation energy of around 1.65 eV. After crossing the barrier, the hydrogen atom moves to the exact on-top site of the nearest-neighbor carbon atom. After that, the hydrogen atom migrates to the next-nearest-neighbor carbon atom in the graphene surface. The second energy barrier is lower than the former, at around 1.29 eV. In total, there are two energy barriers from V211 þ H/Gr to form V111 þ 2H/Gr; the reaction is endothermic, with a sufficiently small energy difference of approximately 0.2 eV between the initial and final states.
Summary The minimum energy pathways and energy barriers for the adsorption and desorption processes of a reversible reaction (V111 þ H2 4 V221) have been investigated and discussed based on DFT calculations. We found the pathways for H2 dissociative chemisorption on the V111 surface, and measured the energy barriers of dissociative chemisorption (V111 þ H2 / V221) and desorption (V221 / V111 þ H2) to be around 1.3 eV. The existence of this comparable energy value for both adsorption and desorption suggests that the local structure of V111 works as a self-catalyst that is created in the graphene structure for both adsorption and desorption processes. In this structure, there are no elements other than hydrogen and carbon. We conclude that the process design of V111 þ H2 4 V221 would be a new strategy for designing a hydrogen storage application. A relevant point in this process is the fact that the local V111 structure has been observed experimentally [50]. After annealing a hydrogenated surface, this fascinating structure was clearly observed [50]. Thus, selective creation of V111 in various graphitic materials may be possible. Molecular hydrogen adsorption on V111 is actually realizable, for which experimental evidence has been reported [52]. The migration of hydrogen atoms from the locally dense adsorbed-hydrogen structure of V221 is expected in real experiments or the fabrication process of hydrogenated graphene. Therefore, recovery of V111 is expected, which will be active for a secondary hydrogenation process. This property of cyclic hydrogenation of V111 is the self-catalytic action. Since the reactivity of atomic hydrogen is much higher than that of molecular hydrogen, the process of storing the hydrogen may be performed by atomic hydrogen treatment. In the procedure, a local graphone structure may be realized. This procedure may be performed in an apparatus or in a welldesigned chemical plant. The desorption of the stored hydrogen in the local graphone structure is expected to be relatively easy. Indeed, as shown in this paper, the reverse migration path from atomic adsorption to a local V221 may be somewhat exothermic. We may then expect a totally exothermic desorption process for the hydrogenatedgraphene-based material, in which the hydrogen release is
Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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enhanced by the V221 / V111 þ H2 process. Therefore, we were able to develop further technological details by the settled reaction path of V111 þ H2 4 V221 as the central catalytically enhanced reversible reaction. This new finding would benefit the search for material-based hydrogen storage technologies.
Acknowledgments The calculations were done in the computer centers of Kyushu University and ISSP, University of Tokyo. G. K. S. gratefully acknowledges scholarship support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (123393). The work is supported by JSPS KAKENHI Grant Nos. JP26400357, JP26107526, and JP16H00914 in Science of Atomic Layers.
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[39] Nordlund K, Keinonen J, Mattila T. Formation of ion irradiation induced small-scale defects on graphite surfaces. Phys Rev Lett 1996;77:699e702. [40] Kelly K, Halas N. Determination of and site defects on graphite using c60-adsorbed {STM} tips. Surf Sci 1998;416(1e2):L1085e9. [41] Hashimoto A, Suenaga K, Gloter A, Urita K, Iijima S. Direct evidence for atomic defects in graphene layers. Nature 2004;430:870e3. [42] Lehtinen PO, Foster AS, Ma Y, Krasheninnikov AV, Nieminen RM. Irradiation-induced magnetism in graphite: a density functional study. Phys Rev Lett 2004;93:187202. € ning O, Bielmann M, [43] Ruffieux P, Melle-Franco M, Gro € ning P. Charge-density oscillation on graphite Zerbetto F, Gro induced by the interference of electron waves. Phys Rev B 2005;71:153403. [44] Yazyev OV, Helm L. Defect-induced magnetism in graphene. Phys Rev B 2007;75:125408. [45] Pereira VM, Lopes dos Santos JMB, Castro Neto AH. Modeling disorder in graphene. Phys Rev B 2008;77:115109. [46] Yazyev OV. Magnetism in disordered graphene and irradiated graphite. Phys Rev Lett 2008;101:037203. [47] Meyer JC, Kisielowski C, Erni R, Rossell MD, Crommie MF, Zettl A. Direct imaging of lattice atoms and topological defects in graphene membranes. Nano Lett 2008;8(11):3582e6. mez-Rodrı´guez JM. [48] Ugeda MM, Brihuega I, Guinea F, Go Missing atom as a source of carbon magnetism. Phys Rev Lett 2010;104:096804. [49] Kondo T, Honma Y, Oh J, Machida T, Nakamura J. Edge states propagating from a defect of graphite: scanning tunneling spectroscopy measurements. Phys Rev B 2010;82:153414. [50] Ziatdinov M, Fujii S, Kusakabe K, Kiguchi M, Mori T, Enoki T. Direct imaging of monovacancy-hydrogen complexes in a single graphitic layer. Phys Rev B 2014;89:155405. [51] Ziatdinov M, Fujii S, Kusakabe K, Kiguchi M, Mori T, Enoki T. Visualization of electronic states on atomically smooth graphitic edges with different types of hydrogen termination. Phys Rev B 2013;87:115427. [52] Kudo Y. Electron transport and near-edge X-ray absorption fine structure study on the effect of structural defects on the electronic structure of graphene [Ph.D. thesis]. Tokyo Institute of Technlogy; 2013. [53] Jiang QG, Ao ZM, Zheng WT, Li S, Jiang Q. Enhanced hydrogen sensing properties of graphene by introducing a mono-atom-vacancy. Phys Chem Chem Phys 2013;15:21016e22. [54] Sunnardianto GK, Maruyama I, Kusakabe K. Dissociationchemisorption pathways of H2 molecule on graphene activated by a hydrogenated mono-vacancy V11. Adv Sci Eng Med 2016;8(6):421e6.
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[55] Martin RM. Electronic structure, basic theory, and practical methods. 2004. Cambridge. [56] Giannozzi P, Baroni S, Bonini N, Calandra M, Car R, Cavazzoni C, et al. Quantum espresso: a modular and opensource software project for quantum simulations of materials. J Phys Condens Matter 2009;21(39):395502. [57] Perdew JP, Zunger A. Self-interaction correction to densityfunctional approximations for many-electron systems. Phys Rev B 1981;23:5048e79. [58] Kohn W, Sham LJ. Self-consistent equations including exchange and correlation effects. Phys Rev 1965;140:A1133e8. € chl PE. Projector augmented-wave method. Phys Rev B [59] Blo 1994;50:17953e79. [60] Vanderbilt D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys Rev B 1990;41:7892e5. [61] Kokalj A. Computer graphics and graphical user interfaces as tools in simulations of matter at the atomic scale. Comput Mater Sci 2003;28(2):155e68. nsson H. Quantum and thermal effects in H2 [62] Mills G, Jo dissociative adsorption: evaluation of free energy barriers in multidimensional quantum systems. Phys Rev Lett 1994;72:1124e7. [63] Henkelman G, Jonsson H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J Chem Phys 2000;113(22):9978e85. [64] Pack JD, Monkhorst HJ. “Special points for brillouin-zone integrations” e a reply. Phys Rev B 1977;16:1748e9. [65] Sanchez-Portal D, Artacho E, Soler JM. Projection of planewave calculations into atomic orbitals. Solid State Commun 1995;95(10):685e90. [66] Ranjbar A, Bahramy MS, Khazaei M, Mizuseki H, Kawazoe Y. First-principles study of structural stability, magnetism, and hyperfine coupling in hydrogen clusters adsorbed on graphene. Phys Rev B 2010;82:165446. [67] Lide D. CRC handbook of chemistry and physics. 85th ed. CRC Press; 2004. [68] Tozzini V, Pellegrini V. Reversible hydrogen storage by controlled buckling of graphene layers. J Phys Chem C 2011;115:25523e8. D. Binding of hydrogen on benzene, [69] Ma J, Michaelides A, Alfe coronene, and graphene from quantum monte carlo calculations. J Chem Phys 2011;134. 134701e1e6. [70] Jeloaica L, Sidis V. Dft investigation of the adsorption of atomic hydrogen on a cluster-model graphite surface. Chem Phys Lett 1999;300:157e62. [71] Sha X, Jackson B. First-principles study of the structural and energetic properties of h atoms on a graphite (0 0 0 1) surface. Surf Sci 2002;496:318e30. [72] Denis PD, Iribarne F. On the hydrogen addition to graphene. J Mol Str Theochem 2009;907:93e103.
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Storing-hydrogen processes on graphene activated by atomic-vacancies Gagus Ketut Sunnardianto a,*, Isao Maruyama b, Koichi Kusakabe a a
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-chou, Toyonaka, Osaka, 560-8531, Japan b Faculty of Information Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro, Higashi, Higashi-ku, Fukuoka, 811-0295, Japan
article info
abstract
Article history:
We investigated the minimum energy pathways and energy barriers of reversible reaction
Received 10 November 2016
(V111 þ H24V221) based upon calculations using density functional theory. We find a
Received in revised form
comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and
11 January 2017
desorption processes. The charge transfer rate from a reacting hydrogen atom to the
Accepted 19 January 2017
graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path
Available online xxx
to recover the initial structure of V111 is realized by the migration of hydrogen atoms from
Keywords:
tion and desorption suggests that this novel storage and release concept has the potential
Graphene vacancy
to act as a hydrogen storage system for certain applications.
V221 onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorp-
Adsorptionedesorption
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Activation barrier
Introduction Nowadays, with the aim of realizing a hydrogen society, much effort is being been directed toward finding solutions to several unresolved problems in hydrogen-based technologies [1e6]. As a relevant subject with the potential to reduce financial costs, innovation is necessary in hydrogen storage for long-distance transport. Organic hydrides (OHs) including methyl-cyclohexane have been considered as a next-stage solution that is now under development [7e11]. However, a problem with OHs is that they are difficult to dehydrogenate, which demands a properly chosen catalyst [12]. Since the discovery of graphene and its remarkable properties [13e15], this material has attracted much attention and is providing promising candidates for carbon-based materials in various fields related to material science. This is
particularly so for the problem of hydrogen storage, for which graphene is regarded as particularly promising [16e27] because of its hydrogenationedehydrogenation reactions. Hydrogenated graphene was first realized by Elias et al. [18], who demonstrated that the hydrogenation reaction is reversible. This characteristic makes hydrogenated graphene a potential candidate for hydrogen storage [20]. Thus, the interaction between graphene and hydrogen is now the subject of much discussion [28e32]. However, there remain some barriers to bring into real applications. It is well known that a hydrogen molecule interacts weakly with pristine graphene. In molecular hydrogen adsorption, it is physisorption that occurs primarily, and the dissociation rate of hydrogen molecules on pristine graphene is very low. This is because the energy barriers against dissociating a H2 molecule on pristine graphene are high, amounting to as much as 2.7e3.3 eV [33,34]. The chemical inertness of a graphene
* Corresponding author. E-mail address: [email protected] (G.K. Sunnardianto). http://dx.doi.org/10.1016/j.ijhydene.2017.01.115 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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surface against molecular hydrogen limits the volume density of hydrogen in the molecular adsorption form [24]. Starting from atomic hydrogen, however, the resultant adsorbed surface is in another form with chemisorbed hydrogen atoms. Atomic hydrogen reacts with the graphene more easily over a wide range of coverage [35]. However, in general, easy adsorption leads to difficult desorption; the desorption energy barrier amounts to 1.4 eV [36]. Thus, although we have sufficient hydrogen density for storage use when chemically hydrogenated graphene is created, we have to solve the problem of the desorption barrier. Previous study, we have succeeded in deriving the rule that determines the charge transfer rate (CTR) from hydrogen to graphene over a wide range of coverage. Our findings reveal that the CTR shows roughly linear behavior from 0.22e for the dilute limit to 0.15e for the half-coverage limit [37]. The structure that appears in the half-coverage limit is called graphone [38]. Thus, different desorption characteristics may be expected when the coverage is controlled. Although desorption would be easier for a higher coverage, we should search for an enhancement effect that would work for every coverage. Accordingly, we seek a suitable method for enhancing the desorption rate from a properly prepared hydrogenated graphene surface. The demand for enhanced reactivity has motivated researchers to modify graphene chemically. A plausible way to increase the interaction strength is by forming a vacancy in the graphene. Recently, vacancy defects in graphene have attracted considerable attention in the field of material science [14,39e49]. Atomic vacancies can be created by irradiating graphene with Arþ ions with typical ion energy of 100 eV and exposure times of 3e4 s [50,51]. Furthermore, hydrogenation treatment of graphene with atomic vacancies creates various types of hydrogenated atomic vacancy [50]. Single-vacancy graphene hydrogenated with molecular hydrogen has been realized experimentally [52]. An effect of the reaction has been observed through the apparent change in the conduction properties with respect to the bias gate voltage [52]. This is evidence for the reactivity of hydrogen molecules at the vacancy sites. In fact, successive reactions of molecules at a vacancy may be realized [53,54]. The authors (GKS, I.M and K.K) reported the plausibility of hydrogen-dissociative adsorption on a graphene monovacancy to form atomic on-top hydrogen at an in-plane graphene surface by migration of a hydrogen atom [54]. After initial hydrogenation of an atomic vacancy to form a hydrogenated graphene monovacancy V11 (a graphene vacancy with two hydrogen atoms at the vacancy edge), dissociative chemisorption of H2 on V11 to form V211 (a hydrogenated graphene vacancy at which two carbon atoms are monohydrogenated and one carbon atom is di-hydrogenated at the vacancy edge) is possible. The activation barrier for dissociative chemisorption of a H2 molecule on graphene mono-vacancy V11 is around 0.5 eV. The diffusion reaction of one hydrogen atom on V211 then makes the hydrogen atom migrate on the graphene surface to form V111 þ H. Here the second barrier to migration may be compensated for by the 2.5-eV reaction energy of the first adsorption reaction [54]. To achieve efficient hydrogen storage, fundamental research such as i) the adsorption and desorption
characteristics, ii) the nature of hydrogen diffusion or migration, and iii) the activation barrier of the adsorption or desorption direction is required by specifying the reaction paths to be investigated. In such a path, the initial and final states have to be identified. The discovery of a relevant path may then lead us to have a concept of the self-catalytic reaction that is selected from all possible paths. The resulting path would answer the question of whether hydrogen uptake occurs endothermically or exothermically, or indeed whether energy release takes place or not during hydrogen desorption. In this newly planned study, we report the comparable values of activation barriers for both adsorption and desorption of one hydrogen molecule on a triply-hydrogenated vacancy V111 of graphene. We consider the characteristic reaction path of V111 þ H2 forming V221, in the latter of which two carbon atoms are di-hydrogenated and one carbon atom is mono-hydrogenated at the vacancy edge. This process is reversible. Interestingly, the adsorption and desorption barriers are comparable at around 1.3 eV. The reaction energy is almost negligible, which is useful for hydrogen storage applications. Our discovery of the reversible reaction V111 þ H2 4 V221 leads us to a novel concept of the self-catalytic property of hydrogenated graphene vacancies. The reaction barrier is reduced by the creation of hydrogenated vacancies, typically as a form of V111. The 1.3-eV energy barrier is valuable for applications because it allows us to protect against unfavorable release of hydrogen at low temperatures and near ambient conditions. Once the system is heated above a few hundred Kelvin, hydrogen may be released from V221. In the summary and discussion part of this paper, we discuss plausible evidence from existing experiments for the lift-off process [52]. Our findings may be used as a foundation and guidance for the innovative design of defective graphene as a material for the optimum storage and release of hydrogen.
Materials and methods The structure models used for the simulation were graphene sheets in a rectangular supercell that consists of 63 carbon atoms. Calculations have been done using density functional theory (DFT) [55] implemented in the QUANTUM ESPRESSO code [56]. Local density approximation by the Perdew-Zunger parametrization was adopted [57,58]. We used the projectoraugmented wave method and an ultra-soft pseudo potential [59,60]. All atomic visualization were created using XCrySDen [61]. The nudged-elastic-band method (NEB) was applied to determine the minimum energy pathway of a reaction path and the energy barrier for both the dissociative-chemisorption and desorption processes [62,63]. The parameters were an energy cut-off of 40 Ry for the plane-wave expansion of the wave function and 400 Ry for the expansion of the augmented charge. The convergence criterion for the structural optimization was that the total absolute value of the inter-atomic force vector should be less than A 104 Ry/a.u. The distance between graphene planes was 10 in order to avoid interaction between layers. A distribution of k-points on a 16 16 1 mesh was used via the MonkhorstePack scheme [64]. Along the reaction pathways, the
Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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€ wdin charge analhydrogen charge was obtained by using Lo ysis [65]. Because hydrogen atoms repel each other geometrically at a vacancy, the hydrogenated vacancy is not planar. We identified a buckled hydrogenated carbon network, the detailed structure of which depends on the orientation of the adsorbed hydrogen atoms. If a hydrogen atom is adsorbed at the top of a carbon atom, the hydrogenated carbon atom buckles out of the plane because of the formation of local sp3-like bondings. To check whether the chosen calculation parameters were sufficiently accurate, we performed structural optimization of single hydrogen adsorption on graphene as well as the V111 structures. The benchmark results showed that the CeH bond length was 1.12 A and that the buckling of graphene was from the shift of an adsorbed carbon atom upward by 0.38 A from the graphene plane. These numerical values are almost the same as those found in available references [17,30,33,35,66e72]. For the V111, the buckling was estimated to be due to an upward shift of hydrogenated carbon of 0.19 A, which again is in reasonable agreement with previous results [42,53].
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Fig. 1 e Minimum energy pathway from V111 þ H2 to V221. The blue and red atoms denote carbon and hydrogen, respectively. Symbols IS, TS, FS, u, d, and sd represent the initial state, transition state, final state, up configuration, down configuration, and slightly down configuration, respectively.
Results and discussion Reaction pathways of H2 dissociative adsorption on the V111 surface We previously reported the reaction of a hydrogen molecule at a V11 surface to form V211 [54]. Here V11 is a graphene vacancy with two hydrogen atoms at the vacancy edge, whereas V211 is a graphene vacancy for which two carbon atoms are monohydrogenated at the vacancy edge and one is dihydrogenated. The energy barrier for this reaction is around 0.5 eV [54]. Moreover, the V211 structure has a reaction path to the V111 with a migrated hydrogen atom on the in-plane graphene [54]. Once the migration reactions occur, the local V111 structure can allow further hydrogenation to form a V221 structure in the molecular hydrogen treatment. Once V221 has formed at a certain hydrogen pressure, we expect a reversible reaction path between V111 þ H2 and V221. Here, we investigate the pathways for H2 dissociation on the V111 surface and its energy barrier (see Fig. 1). Firstly, in the initial state, the adsorbed H2 molecule is located on the hollow site of the vacancy. As the reaction proceeds, the hydrogen molecule (with its bond length of around 0.77 A) rotates a little, causing the energy of the system to increase. The bond elongates to 0.8 A, followed by dissociation of the hydrogen molecule with an energy barrier of around 1.28 eV. In the transition state (TS) as shown in Fig. 1, the H2 molecule dissociates into two H atoms weakly bound to the C atoms at the vacancy edge. After that, each hydrogen atom is adsorbed by two mono-hydrogenated carbon sites, forming V221 (u-ud-ud). Along the pathways, the configuration of V111 (u-d-sd) is changed prior to the dissociation of the hydrogen molecule to be V111 (u-d-d) at the transition state (Figs. 1 and 2). We observed that the energy difference between V111 þ H2 and V221 was relatively small at around 0.03 eV. The energy
barrier for the dissociative chemisorption at the graphene V111 surface was around 1.28 eV. In this case, a single vacancy can be regarded as a “self-catalytic” reaction center. This plays an important role in reducing the energy barrier. The dehydrogenation of V221 is the reverse reaction path of V111 þ H2 / V221, which gives a reaction path to form V111 þ H2 from V221. As the dehydrogenation proceeds, the hydrogen atom in the up orientation in the di-hydrogenated carbon site starts to desorb. Then, in the transition state, two of the desorbed hydrogen atoms start to form a H2 molecule in the vacuum, forming the final states V111 þ H2. It is interesting to note that the reversible reaction of V111 þ H2 4 V221 has comparable energy-barrier values for both adsorption and desorption. The small energy difference between the initial (V221) and final (V111 þ H2) states, this suggests efficient hydrogen release and uptake processes, making the storage device economically feasible. Thus, we reason that the pathways from V111 þ H2 / V221 and its reverse reaction represent good process designs for hydrogen storage. This is also due to the comparable activation-barrier values for both adsorption and desorption, which means that these reversible reactions are more thermodynamically plausible than the other competing materials, namely OHs.
Charge-transfer rate Information on the charge state of hydrogen could lead to a method for controlling the reversible reaction. In order to determine the change in the hydrogen charge along the pathways and the charge transfer rates from hydrogen to graphene, we performed charge analysis to calculate the values of hydrogen charge listed in Table 1.
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Fig. 2 e Structures appearing along the pathway. Left: initial state (IS); center: transition state (TS); right: final structure (FS) of V 221(u-ud-ud). The blue and red atoms denote carbon and hydrogen, respectively. In the transition state, one hydrogen atoms (number one) have changed from sd configuration in the initial state into d configuration in the transition state. Each hydrogen atom is numbered to identify the value of hydrogen charge shown in Table 1. Symbols u, d, and sd represent up, down, and slightly down configurations, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 e Values of hydrogen charge in the initial state V111 þ H2 (IS), transition state (TS), and final state V221 (uud-ud) (FS) of the pathway. The different H atoms are numbered as in Fig. 2. No. 1 2 3 4 5
IS charge [e]
TS charge [e]
FS charge [e]
0.82 0.80 0.81 0.99 0.99
0.81 0.81 0.83 0.89 0.91
0.80 0.80 0.83 0.82 0.82
We found that the charge of a neutral hydrogen molecule in the initial configuration has an almost symmetric distribution with respect to the molecular center, although weak interaction from the hydrogenated vacancy structure inclines the molecular axis toward the surface (see Fig. 3). The partial summation value of the charge is 0.99 e per hydrogen atom. In the transition state, the hydrogen molecule dissociates into two hydrogen atoms, which are numbered 4 and 5 in Fig. 2. The electron charge of hydrogen atom no. 4 decreases to 0.89 e, whereas that of hydrogen atom no. 5 decreases only to 0.91 e. In the final state, the charges of hydrogen atoms nos. 4 and 5 both decrease to 0.82 e. Thus, there is electron transfer from the hydrogen to the graphene mono-vacancy along the reaction path proceeding from the transition state to the final
state. In the final state, we find that the charge transfer from hydrogen to the graphene surface is around 0.18e per hydrogen. In the initial state, the inter-atomic distance d of the reacting atoms of the hydrogen molecule is around 0.77 A, whereas in the transition states an elongation of d ¼ 0.80 A occurs before dissociation of the hydrogen molecule.
Migration of a hydrogen atom from V221 It is possible to have several migration paths of atomic hydrogen on the defective graphene V221. We examined the barrier for diffusion of hydrogen atoms from the vacancy site to the graphene surface, i.e., V221 / V211 þ H/Gr. By looking at this process, we observed hydrogenation both at the vacancy and on the graphene surface. Once V221 was created, there were possible transition paths toward V211 with a migrated hydrogen atom on the graphene surface (H/Gr). In fact, V211 with migrated hydrogen may arise from V221. We observed an activation barrier for the migration of a hydrogen atom from V221 to V211 þ H/Gr. Here, the last hydrogen in V211 þ H/Gr was on top graphene surface, for which the total energy was approximately 0.8 eV higher than that for V221. Firstly, a hydrogen atom in one of the di-hydrogenated carbon sites migrates to the exact on-top site of the nearest bare carbon atom. After that, the hydrogen atom migrates to the next nearest carbon atom in the graphene plane.
Fig. 3 e Top (upper panel) and side (lower panel) views of charge density profiles appearing along the pathway. The electronic density increases from blue to white to red. Blue indicates electron deficit whereas red indicates electron excess. These figures were obtained using the XCrySDen graphical user interface [61]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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Fig. 4 e Minimum energy pathway from V221 to V211 þ H/ Gr, where H/Gr is an adsorbed hydrogen atom on the graphene plane. The blue and red atoms are carbon and hydrogen, respectively. Their atomic structures are given by the insets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This is a two-step reaction path. In the first step, one of the hydrogen atoms in the di-hydrogenated carbon site migrates to the nearest-neighbor bare carbon atom with an energy barrier of 1.57 eV. Subsequently, the migrated hydrogen atom diffuses to the second carbon atom in the graphene surface with an energy barrier of 1.67 eV (see Fig. 4). The energy of the product is higher than that of the reactant, so the migration process from V221 to V211þ (an adsorbed H on the graphene) is an endothermic reaction. Thus, there are two energy barriers for this two-step reaction: the first step requires an energy of 1.57 eV to cross the potential barrier, and the second step requires an energy of 1.67 eV to reach the final state. Once V211 þ H/Gr is realized, there is the possibility of further migration from V211 þ H/Gr to V111 þ 2H/Gr, where 2H/
Fig. 5 e Minimum energy pathway from V211 þ H/Gr to V111 þ 2H/Gr, where H/Gr is an adsorbed hydrogen atom on the graphene plane. Here, 2H/Gr denotes two adsorbed hydrogen atoms on the graphene plane. The blue and red atoms are carbon and hydrogen, respectively. Their atomic structures are given by the insets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Gr denotes two adsorbed hydrogen atoms on the graphene plane. Hence, we observed the migration pathways and the activation barrier of the migration from V211 þ H/Gr to V111 þ 2H/Gr. As shown in Fig. 5, there are again two transition states, both of which must be overcome to reach the final states. Firstly, a hydrogen atom migrates from the dihydrogenated carbon site to the nearest-neighbor bare carbon atom with an activation energy of around 1.65 eV. After crossing the barrier, the hydrogen atom moves to the exact on-top site of the nearest-neighbor carbon atom. After that, the hydrogen atom migrates to the next-nearest-neighbor carbon atom in the graphene surface. The second energy barrier is lower than the former, at around 1.29 eV. In total, there are two energy barriers from V211 þ H/Gr to form V111 þ 2H/Gr; the reaction is endothermic, with a sufficiently small energy difference of approximately 0.2 eV between the initial and final states.
Summary The minimum energy pathways and energy barriers for the adsorption and desorption processes of a reversible reaction (V111 þ H2 4 V221) have been investigated and discussed based on DFT calculations. We found the pathways for H2 dissociative chemisorption on the V111 surface, and measured the energy barriers of dissociative chemisorption (V111 þ H2 / V221) and desorption (V221 / V111 þ H2) to be around 1.3 eV. The existence of this comparable energy value for both adsorption and desorption suggests that the local structure of V111 works as a self-catalyst that is created in the graphene structure for both adsorption and desorption processes. In this structure, there are no elements other than hydrogen and carbon. We conclude that the process design of V111 þ H2 4 V221 would be a new strategy for designing a hydrogen storage application. A relevant point in this process is the fact that the local V111 structure has been observed experimentally [50]. After annealing a hydrogenated surface, this fascinating structure was clearly observed [50]. Thus, selective creation of V111 in various graphitic materials may be possible. Molecular hydrogen adsorption on V111 is actually realizable, for which experimental evidence has been reported [52]. The migration of hydrogen atoms from the locally dense adsorbed-hydrogen structure of V221 is expected in real experiments or the fabrication process of hydrogenated graphene. Therefore, recovery of V111 is expected, which will be active for a secondary hydrogenation process. This property of cyclic hydrogenation of V111 is the self-catalytic action. Since the reactivity of atomic hydrogen is much higher than that of molecular hydrogen, the process of storing the hydrogen may be performed by atomic hydrogen treatment. In the procedure, a local graphone structure may be realized. This procedure may be performed in an apparatus or in a welldesigned chemical plant. The desorption of the stored hydrogen in the local graphone structure is expected to be relatively easy. Indeed, as shown in this paper, the reverse migration path from atomic adsorption to a local V221 may be somewhat exothermic. We may then expect a totally exothermic desorption process for the hydrogenatedgraphene-based material, in which the hydrogen release is
Please cite this article in press as: Sunnardianto GK, et al., Storing-hydrogen processes on graphene activated by atomic-vacancies, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.115
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enhanced by the V221 / V111 þ H2 process. Therefore, we were able to develop further technological details by the settled reaction path of V111 þ H2 4 V221 as the central catalytically enhanced reversible reaction. This new finding would benefit the search for material-based hydrogen storage technologies.
Acknowledgments The calculations were done in the computer centers of Kyushu University and ISSP, University of Tokyo. G. K. S. gratefully acknowledges scholarship support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (123393). The work is supported by JSPS KAKENHI Grant Nos. JP26400357, JP26107526, and JP16H00914 in Science of Atomic Layers.
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