Magnetic Moment Controlling Desorption Temperature in Hydrogen

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Magnetic Moment Controlling Desorption Temperature in Hydrogen Storage: Case of Zr Doped Graphene as High Capacity Hydrogen Storage Media Asha Yadav, Brahmananda Chakraborty, Abhijeet Sadashiv Gangan, Nainesh Patel, Mehernosh R. Press, and Lavanya M Ramaniah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04886 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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The Journal of Physical Chemistry

Magnetic Moment Controlling Desorption Temperature in Hydrogen Storage: Case of Zr Doped Graphene as High Capacity Hydrogen Storage Media A.Yadava, Brahmananda Chakrabortyb*, Abhijeet Ganganc, Nainesh Patela, M.R. Pressa and Lavanya M. Ramaniahb

a

Department of Physics, University of Mumbai, Vidyanagari, Santacruz (E), Mumbai 400 098, India. b

High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India c

Department of Mathematics, Ramnarian ruia college, Matunga , Mumbai-400019, India

ABSTRACT For the first time we predict through Density Functional Theory that a single Zr atom attached on graphene surface can adsorb maximum of 9 H2 molecules with average binding energy of 0.34 eV and average desorption temperature of 433 K leading to a wt% of 11, higher than the DoE’s requirement of 6.5 wt%.The dependency of desorption temperature (TD) of H2 molecule with the magnetic moment (µ) of the system was exclusively studied by formulating the empirical relation TD = T0 + aµ b (with T0 = 399K, a = 302.38 J-1TK and b = 0.5).For system with large magnetic moment the charge transfer to the hydrogen molecule is higher leading to higher desorption temperature (may be higher than prescribed limit for hydrogen storage by DoE). As the magnetic moment reduces, TD comes into the desired window for fuel cell applications.It can be inferred from this study that controlling the magnetic character of the system through doping may be an effective way to bring TD in to the desiredwindow. We qualitatively and extensively demonstrate through the analysis of the partial density of states and Bader charge transfer the interaction mechanism of Zr on graphene surface and hydrogen storage capability of Zr decorated graphene. As we have used GGA exchange correlations (LDA over binds the system), checked the stability through ab-initio MD simulations, computed the diffusion barrier for avoiding metal-metal clustering and predicted hydrogen wt% of the system (11 wt%) comes higher than ACS Paragon Plus Environment


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the DoE’s requirement (6.5 wt%) with desorption temperature (433 K) very much suitable for fuel cell applications we strongly believe that Zr doped graphene can be tailored as high capacity hydrogen storage device.

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Corresponding author: Dr. Brahmananda Chakraborty

E-mail address: [email protected]

1. INTRODUCTION Hydrogen fuel is one of the most promising alternatives to conventional energy sources as it is abundant, environment friendly, has the highest energy density of any chemical fuel,3and its by-products are just clean water and heat. However, the efficient and compact storage of hydrogen is a major challenge for practical implementations of hydrogen fuel as currently employed hydrogen storage technologies, such as high pressure tank and liquid state storage are not suitable, because of the large size and weight of the tank and the high energy costs of liquefaction. Solid state storage would be most efficient and desirable if the storage medium can adsorb large amount of hydrogen1,2and can release them easily without changing its structure. In this respect, the US Department of Energy (DoE)4-6has prescribed the guidelines that for a system to be regarded as an efficient hydrogen storage deviceit should be able to store a minimum of 6.5 wt.% of hydrogen by weight1 and the hydrogen (H2) molecules must have a binding energy in the range of 0.2 - 0.7 eV.7Carbon nanostructures have been explored as hydrogen storage device due to their large surface area and light weight.8-20But unfortunately, for pure carbon nanostructures such as fullerenes, carbon nanotubes and graphene,hydrogen storage is negligible at room ambient condition.21 It has been reported that the transition metals - decorated carbon nanostructures are promising candidates for hydrogen storage media at ambient conditions, as the metals can be bonded strongly to the nanostructure and are also able to bind H2 molecules at room temperature.22,23Yildirim et al.found using density functional theory (DFT), that a single Ti atom decorated on single walled carbon nanotubes (SWCNT) can adsorb four H2 molecules, where three are in molecular form and one H2 molecule gets dissociated.15Earlier, we had presented24a comparative study on bonding mechanism and hydrogen adsorption capabilities of various transition metals (TM) atoms decorated on SWCNTs and concluded that TMs with minimum number of d ACS Paragon Plus Environment

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electrons in the outermost shell are better hydrogen absorbers.We had also reported through first principles simulations that a single Yttrium (Y) atom on SWCNT can physisorb up to 6 H2 molecules with 100% desorption, leading to 6.1 wt% of hydrogen.25Hydrogen storage capability of various 3d TMs, e.g., Ti, Sc, V and Cr on C60and Ni, on carbon nanotube (CNT) nanostructures have also been reported.16,17,26Compared to nanotubes and fullerene, 2D systems such as graphene can be better hydrogen storage media as both sides of the surface can be utilized. Hydrogen storage capability of TM-doped graphene has also been studied theoretically.22,27,28 In addition to TMs, alkaline earth metal decorated graphene, SWCNTs and fullerenes have also been reported as promising hydrogen storage media.18,29-30Beheshti Elham et. al have investigated the hydrogen storage capacity of Ca decorated on B-doped graphene and predicted thatB doping improves the binding energy of Ca on graphene 31. So, from literature review, we can see that so far researchers have mostly concentrated on Sc, Ti, Y and Ca doped carbon nanostructures for hydrogen storage but not much attention has been focused on Zr which is just below Ti in the periodic table. In the earlier study, we have seen that for Zr decorated SWCNT first hydrogen molecule gets dissociated and not available for recycling as in the case of Ti decorated SWCNT. However, with graphene the situation may be quite different as there is no curvature effect in graphene contrary to SWCNT. Therefore, it would be quite interesting to explore the hydrogen storage capability of Zr decorated graphene. There are several theoretical reports in the literature, which predicted higher H2 wt% using LDA exchange correlations, but practically achievable wt% was found to be much lower as LDA overbinds as compared to GGA. Another important issue is the stability of the system. For practical implementation of TM-doped CNT as hydrogen storage device it should be stable up to higher temperatures (higher than maximum desorption temperature), there should not be metal-metal clustering and the adsorbed hydrogen should not desorb from the system at room temperature. We have taken care of these practical issues very carefully and checked the stability of the system through abinitio molecular dynamics (MD) simulations and computation of diffusion energy barrier for metal-metal clustering. As we have considered GGA exchange correlations along with van der Waal’s corrections for computinga more accurate binding energy, we believe that our predicted H2 wt% should be close to the experimental value. Even though the hydrogen storage by TM - doped carbon nanostructures has been considerably studied in the literature, the important fact that the role of magnetic moment of the host (metal + carbon nanostructure) can play an important rolein controlling the desorption temperature of the H2 molecules has not been explored so far. Astonishingly, there are studies on hydrogen storage by TM - doped CNTs,32 where non-spin polarization calculations only have been carried out. As clarified in our previous reports,

24,25

non-spin polarization

calculations yield different binding energies and adsorption site preference, compared to spin polarized calculations, which must therefore be considered24as TM –doped carbon nanostructures possess magnetic signature. Magnetic moment of metal + carbon nanostructure can control charge transfer from metal to ACS Paragon Plus Environment


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hydrogen molecule and hence indirectly control the binding energy of the adsorbed H2 molecules. As desorption temperature is proportional to binding energy as per Vant’s Hoff equation25, there must exist a relation between magnetic moment of the host and desorption temperature of hydrogen molecules which need to be explored. Here, we report detailed DFT investigations on hydrogen storage capability of Zr - doped graphene sheet and predict that a single Zr atom bonded on graphene surface can adsorb maximum of 9 molecular H2 with average binding energy of 0.34 eV and average desorption temperature of 433 K. Zr loaded on alternate hexagons on both surface of graphene leads to a wt% of 11, higher than the DoE’s requirement of 6.5 wt%. AbinitioMD simulations carried out at 300 K and 900 K showed that the system is stable at higher temperature and Zr atoms do not cluster or desorb fromthe system even at 900 K. We have also relate the magnetic moment of the host with the desorption temperature of H2 molecules and formulate an empirical relation between them.

2. COMPUTATIONAL DETAILS Density Functional Theory (DFT) based projector augmented wave (PAW) method, as implemented in the VASP code, was employed to carry out all spin polarized calculations including MD simulations33-36. A hexagonal 4x4 graphene layer supercell containing 32 carbon atoms was considered for all calculations. As VASPcodeemploys periodic boundary conditions, in order to avoid the interaction between adjacent periodic images a vacuum of 15 Å was inserted in z-direction perpendicular to the graphene plane. In view of the fact that LDA overestimates binding energy of the systems37,all the calculations are carried out using GGA for the exchange correlation term. Here in PAW potentials, 4d2 5s2, 2s2 2p2 and 1s1 states for Zr, C and H atoms, respectively, were chosen for valence configurations.The plane-waveenergy cut-off was chosen to be 500 eV.A Monkhorst-Pack grid of 15x15x1 k- points was used to sample the Brillouin zone38. The self-consistent field convergence threshold was taken to be 10-5 eV, while the Hellmann-Feynman forces for structural relaxation were less than 0.01eV/Ǻ. Structural relaxation and total-energy calculations were carried out with Zr on graphene surface to study the metal-organic interactions and the step-wise adsorption of H2 molecules. As DFT does not include the weak dispersion forces, the optimizations and total energy calculations were repeated using Grimme’s DFT-D2,39 which uses a pair-wise force field optimized for the GGA exchange-correlation functional for describing the van der Waals interactions.To check the stability of the system at higher temperature and whether there is clustering between the Zr atoms at neighbouring sites, a series of abinitio MD simulations at 300K and 900K were carried out within the canonical NVT ensemble with Γ-point sampling for Brillouin zone integration and 0.5 fs time-step anda Nośe thermostat for temperature control. For simulation at 300 K, first temperature was increased from 0 K to 300 K for 3 ps in NVE ensemble and then system was heated at constant temperature of 300 K in NVT ensemble for 2.5 ps. Similarly, for 900 K calculation, initially the temperature ACS Paragon Plus Environment

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was increased from 0 K to 300 K in NVE ensemble for 3 ps and then from 300 K to 900 K in NVE ensemble for 6 ps and finally it was heated at 900 K in NVT ensemble for another 2.5 ps. 3. RESULTS AND DISCUSSION 3.1 Interaction of Zr on Graphene First we have relaxed the structure of pristine graphene as displayed in Figure 1 with C-C bond length equal to 1.41Å, matching very well with the literature value of 1.42 Å40Figure 1displays the density of states of pristine graphene exhibiting theDirac point at the Fermi level,in consistent with the literature41. Then we have introduced one Zr atom at a distance of 2.5 Å above the grapheme layer and allow the system to relax. Zr atom prefers to stay on the hollow site and make bonds with six carbon atoms on the ring similar to the reports made for Ti and Y decorated graphene layer 42,43. The average C-Zr bond length is 2.44 Å and C-C bond length in that hexagon increases marginally from 1.41Å to 1.43 Å. The binding energy of Zr on graphene surface has been computed using the formula25

Eb ( Zr ) = E (G) + E ( Zr ) − E (G + Zr ) ………………………………………………………………………...….(1) where E(G) and E(Zr) are the energies of the graphene sheet and the isolated Zr atom, respectively, and

E(G + Zr) is the total energy of Zr-doped graphene. The computed binding energies of Zr atom, using the GGA exchange correlation functional comes out to be 2.4 eV, which is higher than the binding energy of Ti (1.09 eV) and Y(1.40) on graphene surface42,43. A BE of 2.4 eV for Zr on graphene surface may be sufficient to avoid desorption of Zr from graphene surface in ambient conditions and at higher temperature during desorption of H2 molecule from Graphene+Zr system. Figure 1 portrays the Spin polarized total density of states (DOS) of pristine graphene layer and Zr decorated at hollow site of graphene layer. Semi-metallic graphene becomes metallic with introduction of Zr atom and interestingly Zr switch on magnetic signature on non-magnetic graphene with a magnetic moment of 3.0µB.Here we note that although isolated Zr has a magnetic moment of 2.0 µB, in graphene its magnetic moment has increased to 3.0 µB, contrary to the case of Zr - decorated SWCNTs, in which the magnetic moment remains 2.0 µB. We have also calculatedmagnetic moment of Y and Ti, which shows increase from 1.0 and 2.0 µB to 2.0 and 3.0 µB, respectively, when attached on graphene surface47. Figure 2 displays the Partial density of states (PDOS) of Zr d, p and s orbital when it is attached on graphene layer (upper panel) and in isolated case (lower panel). For isolated Zr atom, there are states of d and s orbitals on and around Fermi level, which also appear in the case of Zr- decorated graphene making the system metallic.

3.2 Hydrogen Adsorption on Zr Doped Graphene ACS Paragon Plus Environment


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In order to explore the hydrogen storage capacity of Zr doped graphene, first a single H2 molecule is placed at 2.5 Å above the Zr atom and the relaxation and total energy calculations are carried out. The first H2 molecule is found to be adsorbed with a binding energy of 0.66 eV with slight elongation of H-H distance from 0.75 Å to 0.87 Å and having Zr-H distance of 2.045 Å. Although the binding energy of first H2 is slightly higher than the desirable binding energy for hydrogen storage, interestingly, the H2 molecule does not dissociate in contrary to the case of Ti decorated single walled carbon nanotube (SWCNT) and Zr decorated SWCNT15,25. We systematically introduce more and more hydrogen molecules and carry out the relaxation and total energy calculations. It was found that a single Zr atom, attached on graphene surface can adsorb maximum of 9 H2 molecules as shown in Figure 3. Here we mention that the average H-H distance of initial four H2 molecules added after first H2 molecule become 0.846 Å from initial value of 0.75 Å with average Zr-H distance of 2.05 Å. For last 4 H2 molecules average H-H distance is 0.76 Å with average Zr-H separation of 3.72 Å. The average binding energy comes out to be 0.34 eV, which is is in the favourable energy window for hydrogen storage as prescribed by DoE.The desorption temperature of H2 molecule adsorbed on Graphene+Zr system was calculated using Van’t Hoff equation as given below25: Td =(Eb/kB) (∆S/R- ln P)-1 …………………………………………………………………………………..….(2) Where Eb = binding energy, kB= Boltzmann constant, P = pressure (P = 1 atmosphere), R = gas constant and ∆S is change in H2 entropy from gas to liquid phase. The average desorption temperature corresponding to average binding energy of 0.34 eV comes out to be 433 K, which is ideal for fuel cell applications. The binding energy and desorption temperature of successively added H2 molecule is presented in Table 1 along with magnetic moment of the system. We can see from Table 1 that as successive hydrogen molecules are adsorbed on the system the magnetic moment decreases and binding energy of H2 molecule reduces. As all the hydrogens attached to Zr are in molecular form, it is important to include Van der Waals interactions. So we have repeated the optimizations and total energy calculations with the Van der Waals interactions. There is not much change with inclusion of Van der Waals interactions except slight increase in binding energy of H2 molecules. 3.3 Ab initio MD Simulations for Stability of the System For practical implementation of the system (Graphene+Zr) as hydrogen storage device, it should be stable at higher temperatures (maximum desorption temperature). The Zr metal should not move much or form clusters with other metal atoms at higher temperature. The hydrogen molecules also should not desorbfrom the system at room temperature. Another crucial aspect is the loading pattern of Zr on graphene surface, which determines the wt% of hydrogen. The Zr metal atom should be loaded in such a way that maximum H2 molecules should be adsorbed without metal-metal cluster formation. It was reported that Ti and Y form clusters when they are placed on neighbouring hexagons of SWCNT and graphene25, 15.So we have loaded Zr on alternate hexagons to avoid clustering. Zr can also be attachedon the upper and lower surface of graphene plane as the repulsive ACS Paragon Plus Environment

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interaction between the adsorbed Zr cations on the opposite surfaces of graphene hinders the cluster formation. Therefore, the optimum loading would be Zr atom on alternate hexagons on both side of the surface, with single Zr atom adsorbing 9 H2 molecules. In order to check the stability and possible cluster formation of Zr atoms on graphene surface at higher temperature, a set of ab initio MD simulations were carried out at 300 K and 900 K (highest desorption temperature) by putting Zr atoms on different sites, as well as on both the surfaces. Figure 3(b), 3(c) and 3(d) displays thesnap sorts from ab initio MD simulations after 2.5 ps at 300K and 900K. We can notice that the structure remains stable even at 900 K and only small fluctuation in Zr-C bondlength was observed. 3.4Computation of Diffusion Energy Barrier for checking Metal-Metal Clustering The reasons for stability of Zr at the hollow site of graphene layer even at 900 K and absence of clustering of Zr at this temperature was understood by computing the diffusion energy barrier for Zr atom to move from one hollow site to next adjacent unoccupied hollow site. We can notice from Figure 4 that there exists a large energy barrier of 1.23 eV, which may prevents Zr atom from clustering. In order to support it we have calculated the thermal energy corresponding to temperature of 900 K. The thermal energy is given by

Ethermal = 32 KT

.......................................................................................... (3)

where K = Boltzmann constant and T = temperature in Kelvin. The thermal energy corresponding to temperature of 900 K comes out to be 0.117 eV. As the thermal energy is quite less than the diffusion energy barrier it is unlikely for Zr atom to cross the barrier although there exists a small probability considering Maxwell distribution. The stability of the structure up to 900 K, and the existence of a large diffusion barrier, ensures the feasibility of the system as a hydrogen storage medium.The wt% computed considering Zr atom at alternate hexagons on both sides of the grapheme planes with 9 H2 moleculesper Zr atoms comes out to be 11.5 wt%, much higher than DoE requirement of 6.5 wt%. Here we mention that theoretical calculations without detailed checking of stability and clustering issues through ab initio MD simulations and computation of diffusion energy barrier may give a misleadingly high wt%, which cannot be reproduced experimentally due to clustering in real systems.27,44Sincewe have consideredGGA + van der Waals corrections (LDA overestimates wt%), established the stability through ab initio MD simulations and diffusion energy barrier calculations, and considered the Zr loading pattern allowing practical feasibility, we strongly believe that our computed hydrogen wt% would be very close to the experimental value. 3.5 Partial Density of States and Bader Charge Transfer analysis of Graphene+Zr In this section, we will investigate the interaction mechanism of Zr on graphene surface and get insight through electronic structures for hydrogen storage capability of Zr decorated graphene in terms of Partial Density of ACS Paragon Plus Environment


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States and Bader charge transfer analysis. For isolated Zr atom, asymmetric nature of 5s and 4d states near the Fermi level as displayed in Figure 2 makes Zr atom magnetic with a magnetic moment of 2µB. On introduction of Zr on graphene surface there is redistribution of charge which alters the electronic and magnetic character of the system. Figure 5(a) and 5(b) presents Partial Density of States of d sub-orbitals of Zr before and after attaching on graphene surface. We can notice that for isolated Zr atom charge stays on dz2 and dx2-y2sub orbitals in addition to s orbital. When Zr is bonded on graphene surface the magnitude of Partial Density of States of occupied dz2 and dx2-y2sub orbitals reduces a lot indicating charge transfer from these sub orbitals to C p orbital. Here we note that there appear some occupied states in dxysub orbital as a result of charge redistribution in graphene+Zr system. In addition to charge transfer from Zr d orbital, interestingly there is also charge transfer from 5s orbital to C p orbital42 as 5sspin down statesmove towards higher energy and become unoccupied. The charge transfer in one spin channel, i.e., spin transfer may be the reason for increase in magnetic moment from 2.0 µB in isolated Zr to 3.0 µB in Zr doped graphene. So 5s orbital of Zr also plays a crucial role in charge transfer and bonding mechanism in addition to Zr d orbital. Figure 6 displays Partial Density of States of carbon p orbital for pristine graphene, bonded and non-bonded carbon atoms when Zr is adsorbed on graphene. The presence of additional pz states on and around Fermi level indicates charge gain from Zr. There is strong hybridization between carbon pz and Zr d orbital as seen from Figure 6(C) and 6(d). Here we mention that there is not much change in Partial Density of States of pxand py sub orbital signifying that only pz π bonds are involved in bonding of Zr on graphene surface. Interestingly, due to charge gain by C p orbitals there is spinsplitting demonstrating that bonded C p orbitals also contribute to the magnetisation of the system. The charge transfer was also confirmed by Bader charge analysis which shows that there is anet charge loss of 1.24e by Zr atom. 3.6 Bonding and Charge transfer Mechanism of H2on Graphene+Zr

Once first H2 molecule is introduced there is charge transfer from Zr d orbital to H s orbital as evident from the reduction of Partial Density of States of Zr d orbitals in Graphene+Zr+H2 system in compared to Graphene+Zr in Figure 7(a) and 7(b). Figure 5(b) and 5(c) displays thePartial Density of States of d sub-orbitals of Zr before and after adsorption of first H2 molecule. A careful inspection of these two plots reveal that for dz2, the intensity of occupied states in the valence band close the Fermi level reduces pointing towards charge transfer from dz2to Hs orbital. Also, the reduction in the intensity of occupied dx2-y2 sub orbital near Fermi level suggests a charge donation from dx2-y2to Hs orbital. Due to this charge transfer there is an elongation of H-H bond length from 0.75 Å to 0.87 Å.Interestingly, the intensity of dxysub-orbitals near Fermi level increases upon adsorption of first H2 molecule which may be due to back-donation of charge from occupied H s orbital. The Partial Density of States of molecular hydrogen before and after attachment on Graphene+Zr system is presented in Figure 7(c) and 7(d). We can see in Figure 7(d),the molecular bonding states at 8 eV below the Fermi levelwhich may bedue to charge gain from Zr dz2and dx2-y2sub-orbital and reduction of states near Fermi ACS Paragon Plus Environment

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level as a result of charge given to dxysub-orbitals. This donation and back donation of charges between Zr d orbitals and H2 molecule indicates Kubas type interaction45,46 which is responsible for higher binding energy of H2 molecule compared to binding due to just physisorption. We have also carried out the Bader charge analysis and found that the net charge loss by Zr atom is 0.4e.Since charge gain by H-atoms of H2 molecule is not sufficient to break the H-H bond, it remains in molecular form with elongation in bondlength of 0.87 Å. As the system adsorb more and more H2 molecules there is net charge transfer from Zr d orbital and the Density of States for Zr d orbital at Fermi level reduces as see in the Figure 8.Bonding mechanism between C, Zr and H2 molecules was further studied in detail through plane waves projected pseudo-atomic orbitals as displayed in Figure 9. From the Figure 9a, it is clear that Zrdx2-y2and dz2sub-orbitals hybridize with C pz orbital. On the addition of first H2, there is hybridization between H 1s and Zr dz2as displayed in Figure 9(b),which is also consistent with our PDOS analysis thatnet charge donation is from Zrdz2 to H 1s.This hybridization is also clearly seen from PDOS as shown in Figure 10a, which shows the interaction of Zrdz2 with antibonding orbital of H2.Another interesting results is obtained by looking at the orbital overlapping in Graphene+Zr+5H2 which clearly indicates that, when H2 was approached from the top of Zr, H1s orbital strongly overlaps with Zrdxz (shown in Figure 9c), whereas when H2 was approached from side, H1s interacts strongly with dxy (shown in Figure 9d) which is in agreement with PDOS shown in Figure 10b and 10c. This predicts that site of adsorptions plays an important role in H2 binding. 3.7 Relation between Magnetic Moment & Desorption Temperature Even though the hydrogen storage by TM metal doped carbon nanostructures has been studied by many researchers, the role of magnetic moment of the system (carbon nanostructures +metal) has not been explored so far. In fact, there are studies on hydrogen storage by TM metal doped carbon nanotubes,24with non-spin polarization calculations.We haveclarified in our previous reports,24,25 that non-spin polarization calculations yield different binding energies and adsorption site preference, compared to spin polarized calculations, which should be considered for accurate description of the system.24Here we try to correlate for the first time the magnetic moment of the system with desorption temperature ofH2 molecules. We can notice from Table 1 that as the system adsorb more and more H2 molecules the magnetic moment of the system as well as binding energy of the H2 molecules reduces. The magnetic moment of graphene+Zr system reduces from 3.0µBto 2.0 µBupon adsorption of first H2 molecule, then further reduces to 0.42 µB with the adsorption of five H2 molecules and finally to 0.30 for 9H2. The reduction in magnetic moment is qualitatively supported through the real-space plot of the effective spin density obtained by taking the difference between the charge densities of majority and minority spins (∆ρ =ρ↑ - ρ↓) as depicted in Figure 11 for iso-value of 0.1e. We can notice from the figure that the total area of the iso-surface which presents a qualitative indication of the magnetic moment reduces as the system adsorb more and more H2 molecules. Bader charge analysis predicts that the net charge gain by the hydrogen molecules reduces as the system adsorb more and more hydrogen molecules and becomes less and ACS Paragon Plus Environment


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less magnetic. So when the system has more magnetic moment it transfers more net charges to the H2 molecules leading to higher binding energy. Once the system becomes less magnetic or non-magnetic, the charge transfer will be less resulting lower binding energy. For a non-magnetic system charge transfer and binding energy is expected to be uniform. So the binding energy of the H2 molecules adsorbed on TM doped carbon nanostructures depends on the magnetic signature of the system. As the desorption temperature is related to the binding energy of H2 molecules as given in equation.(2) we can predict for the first time that the magnetic moment of the system controls the desorption temperature of H2 molecules. In Figure 12 we have plotted the desorption temperature of successive H2 molecules with the magnetic moment of the system. It is clear that the desorption temperature increases with the magnetic moment of the system. We formulate an empirical formula to express desorption temperature TD in terms of magnetic moment µB of the system given by equation (3)

TD = T0+aµb…………………………..……(3) whereT0= 399 K, a = 302.38 J-1TK and b =0.5 For the first H2 molecule the desorption temperature is quite high due to its high magnetic moment. From this study, we can infer that if the system can be made less magnetic or non-magnetic through doping the high desorption temperature can be lowered in to the desirable window for ideal fuel cell applications The only limitation of this work is that desorption temperature of first H2 molecule is very high and not available for recycling. If we exclude the first H2 molecule and consider only 8 H2 per Zr then also hydrogen storage capacity comes out to be 10 wt.%, which is higher than DoE’s requirement. 4. CONCLUSIONS We have explored the hydrogen storage capability of Zr doped graphene system using Density Functional Theory simulations and found that a single Zr atom attached on graphene surface can adsorb 9 H2 molecules with average binding energy of 0.34 eV and average desorption temperature of 433 K resulting to a wt% of 11, higher than the DoE’s requirement of 6.5 wt%.There is strong hybridization between carbon p and Zr d orbital due to charge transfer from Zr dz2and dx2-y2 sub-orbitals to carbon p orbital resulting a strong bonding of Zr on grapheme surface with binding energy of 2.4 eV Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and Zr atom does not come out of the system even at 900 K. For the first time we have relate the desorption temperature (TD) of H2 molecule with the magnetic moment (µ) of the system with the empirical relation TD = T0 + aµ b (with T0= 399 K, a = 302.38 J-1TK and b =0.5). We conclude that magnetization controls the desorption temperature of hydrogen molecule and changing the magnetic moment of the system through doping may be an effective way to bring TD to the desired window. 5. AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding author: Dr. Brahmananda Chakraborty E-mail address: [email protected] Phone: +91-2225592057 Notes The authors declare no competing financial interest.

6. ACKNOWLEDGEMENTS BC, LMR, AY and AG thank Dr. N.K. Sahoo for support and encouragement. We also acknowledge the BARC computer centre where this work was carried out, and thank the staff for their help. B.C. thanks Dr. S. Banerjee for his inspiration and useful scientific discussions.N. Patel acknowledges UGC for providing financialsupport through Faculty recharge program. 7. REFERENCES 1. U.S. Department of Energy. http://www.energy.gov 2. Internation Partnership for Hydrogen and Fuel cells in the Economy. http://www.iphe.net 3. U.S.

Department

of

Energy

H2

Program.

https://www.hydrogen.energy.gov/pdfs/doe_fuelcell_factsheet.pdf (accessed October, 2006) 4. Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of Hydrogen in Single-Walled Carbon Nanotubes. Nature1997, 386, 377. 5. Ding, F.; Lin, Y.; Krasnov, P. O.; Yakobson, B. I. Nanotube Derived Carbon Foam for Hydrogen Storage. J. Chem. Phys. 2007, 127, 164703. 6. Lee, S. Y.; Park, S. J. Influence of the Pore Size in Multi-walled Carbon Nanotubes on the Hydrogen Storage Behaviors. J. Solid State Chem. 2012, 194, 307−312. 7. U,S. Department of Energy. http://energy.gov/eere/fuelcells/hydrogen-storage 8. Pupysheva, O. V.; Farajian, A. A.; Yakobson, B.I. Fullerene Nanocage Capacity for Hydrogen Storage. Nano Lett. 2008, 8, 767-774.

ACS Paragon Plus Environment


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9. Patchkovskii,S.; Tse, J.S.; Yurchenko, S.N.; Zhechkov, L.; Heine, T.; Seifert,G. Graphene Nanostructures as Tunable Storage Media for Molecular Hydrogen.. Proc.Natl.Acad. Sci.U.S.A.2005, 102,10439-10444. 10. Muniz, A.R.; Singh, T.; Maroudas, D. Effects of Hydrogen Chemisorption on the Structure and Deformation of Single-Walled Carbon Nanotubes. Apl. Phys. Lett. 2009,94, 103108-103110. 11. Tada, K.; Furuya, S.; Watanabe, K. Ab Initio Study of Hydrogen Adsorption to Single-walled Carbon Nanotubes.Phys. Rev. B2001, 63, 155405-155408. 12. Li, J.; Furuta, T.; Goto, H.; Ohashi, T.; Fujiwara, Y.; Yip, S. Theoretical Evaluation of Hydrogen Storage Capacity in Pure Carbon Nanostructures. J. Chem. Phys.2003, 119, 2376-2385. 13. Hirscher, M.; Becher, M.; Haluska, M.; Dettlaff-Weglikowska, U.; Quintel, A.; Duesberg, G.S.; Choi, Y.M.; Downes, P.; Hulman, M.; Roth, S.; Stepanek, I.; Bernier, B. Hydrogen Storage in Sonicated Carbon Materials. Apl. Phys. A: Mater. Sci. Process. 2001, 72, 129-132. 14. Dagani, R. Tempest in a Tiny Tube Chem. Eng. News 2002, 80, 25-28. 15. Yildirim, T.; Ciraci, S. Titanium-Decorated Carbon Nanotube as a Potential High –Capacity Hydrogen Storage Medium. Phys. Rev. Lett.2005, 94, 175501-175504. 16. Durgun, E.; Ciraci, S.; Yildirim,T. Functionalization of Carbon-based Nanostructures with Light Transition-metal Atoms for Hydrogen Storage. Phys. Rev. B2008, 77, 085405-085413. 17. Yildirim, T.; Iniguez, J.; Ciraci, S. Molecular and Dissociative Adsorption of Multiple Hydrogen Molecules on Transition Metal Decorated C60. Phys. Rev. B2005, 72, 153403-153406. 18. Ataca, C.; Aktürk, E.; Ciraci, S. Hydrogen Storage of Calcium Atoms Adsorbed on Graphene: FirstPrinciples Plane Wave Calculations. Phys. Rev. B2009, 79, R041406-041409. 19. Sankaran, M.; Viswanathan, B. The Role of Heteroatoms in Carbon Nanotubes for Hydrogen Storage. Carbon 2006, 44, 2816-2821. 20. Chen P, Wu X, Lin J, Tan KL. High H2 Uptake by Alkali-Doped Carbon Nanotubes Under Ambient Pressure and Moderate Temperatures. Science 1999, 285, 91-93.


Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Yurum, Y.; Taralp, A.; Veziroglu, T. N. Storage of Hydrogen in Nanostructured Carbon Materials, Int. J. Hydrogen Energy 2009, 34, 3784-3798.

22.Cabria, I.; Lopez,́ M. J.; Alonso, J. A. Enhancement of Hydrogen Physisorption on Graphene and Carbon Nanotubes by Li Doping. J. Chem. Phys.2005, 123, 204721.

23.Liu, Y.; Brown, C. M.; Neumann, D. A.; Geohegan, D. B.; Puretzky, A. A.; Rouleau, C. M.; Hu, H.; Styers-Barnett, D.; Krasnov, P. O.; Yakobson, B. I. Metal-Assisted Hydrogen Storage on Pt-Decorated Single-Walled Carbon Nanohorns. Carbon2012, 50, 4953−4964.

24.Modak, P.; Chakraborty, B.; Banerjee, S. Study on the Electronic Structure and Hydrogen Adsorption by Transition Metal Decorated Single Wall Carbon Nanotubes. Journal of Physics: Condensed Matter, 201224, 185505.

25.Chakraborty, B.; Modak, P.; Banerjee, S. Hydrogen Storage in Yttrium-Decorated Single Walled Carbon Nanotube. J. Phys. Chem. C 2012, 116, 22502−22508.

26.Lee, J. W.; Kim, H. S.; Lee, J. Y.; Kang, J. K. Hydrogen Storage and Desorption Properties of NiDispersed Carbon Nanotubes, Appl. Phys. Lett. 2006, 88, 143126. 27. Chen, C.H.; Chung, T.Y.; Shen, C.C.;Yu, M.S.; Ts C.S.; Shi, G.N.;Huang C.C.;Ger, M.D.; and Lee, W.L. Hydrogen Storage Performance in Palladium-Doped Graphene/Carbon Composites.Int. J. Hydrogen Energy, 2013 ,38, 3681–3688.

28.Ma, L.P.; Wu, Z.S.; Li, J.; Wu, E.D.; Ren, W.C.”; Cheng, H.M. Hydrogen Adsorption Behavior of Graphene Above Critical Temperature. Int J Hydrogen Energy2009, 34 ,2329-32.

29.Chandrakumar, K. R. S.; Ghosh, S. K.Alkali-Metal-Induced Enhancement of Hydrogen Adsorption in C60 Fullerene: An ab Initio Study, Nano Lett.2008, 8, 13-19.

30.Srinivasu, K.; Chandrakumar, K. R. S.; Ghosh, S. K. Computational Investigation of Hydrogen Adsorption by Alkali‐Metal‐Doped Organic Molecules: Role of Aromaticity, Chem. Phys. Chem.2009, 10, 427-435.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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31.Beheshti, E.; Nojeh, A.; Servati, P.A first-principles Study of Calcium-Decorated, Boron-Doped Graphene for High Capacity Hydrogen Storage. Carbon2011, 49, 1561-1567. 32. Froudakis, G.E.Why Alkali-Metal-Doped Carbon Nanotubes Possess High Hydrogen Uptake.Nano Lett. 2001, 1, 531–533. 33. Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B1993, 47, R558561. 34. Kresse, G.; Hafner, J. Ab initio Molecular-Dynamics Simulation of the Liquid-metal–Amorphoussemiconductor Transition in Germanium. Phys. Rev. B1994, 49, 14251-14269. 35. Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis set. J. Comput. Mater. Sci.1996, 6, 15-50. 36. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab Initio Total-energy Calculations Using a Plane-wave Basis set. Phys. Rev. B1996, 54, 11169-11186. 37. Lugo-Solis, A.; Vasiliev, I. Ab initio Study of K Adsorption on Graphene and Carbon Nanotubes: Role of Long-range Ionic Forces. Phys. Rev. B 2007, 76, 235431. 38. Monkhorst, H.J.; Pack, J.D. Special Points for Brillouin-zone Integrations. Phys. Rev. B1976, 13, 5188. 39. Grimme, S. J. Semi Empirical GGA-type Density Functional Constructed with a Long-range Dispersion Correction, Comput. Chem. 2006, 27, 1787-1799. 40. Pujari, B. S.; Gusarov, S.; Brett, M.; Kovalenko, A. Single-Side-Hydrogenated Graphene: Density Functional Theory Predictions, Physical Review B 2011, 84, 041402. 41. Ma, L.; Zhang, J. M.; Xu, K. W.; Ji, V. Hydrogen Adsorption and Storage on Palladium-Decorated Graphene with Boron Dopants and Vacancy Defects: A First-principles Study. Physica E 2015, 66, 4047. 42. Liu, Y.; Ren, L.; He, Y.; Cheng, H. P. Titanium-decorated Graphene for High-Capacity Hydrogen Storage Studied by Density Functional Simulations. J. Phys.: Condens.Matter 2010, 22, 445301. 43. Liu, Wenbo.; Liu, Yang.; Wang. Rongguo. Prediction of Hydrogen Storage on Y-decorated Graphene: A Density Functional Theory Study. Appl. Surf. Sci.2014, 196, 204-208. ACS Paragon Plus Environment

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44. Zhou, H.; Liu, X.; Zhang, J.; Yan, X.; Liu, Y.; and Yuan, A. Enhanced Room-temperature Hydrogen Storage Capacity in Pt-loaded Graphene Oxide/hkust-1 Composites, Int. J. Hydrogen Energy, 2014, 39, 2160–2167. 45. Kubas, G. J. Metal–dihydrogen and σ-bond Coordination: the Consummate Extension of the Dewar– Chatt–Duncansonmodel for Metal–olefin π Bonding, J. Organomet. Chem., 2001, 635, 37-68. 46. Kubas, G.J. Hydrogen Activation on Organometallic Complexes and H2 production, Utilization, and Storage for Future Energy, J. Organomet. Chem., 2009, 694, 2648-2653. 47. Desnavi, S.; Chakraborty B.; Ramniah L. M. First Principles DFT Investigation of Yttrium-doped Graphene: Electronic Structure and Hydrogen Storage, AIP Conf. Proc. 2014, 1591, 1775-1777.

FIGURE CAPTIONS Figure 1: Total DOS of (a) Zr-decorated grapheneand (b) pristine graphene, along with their geometrically

optimized structures, respectively. Here, pink spheres and bluish green spheres correspond to C and Zr atoms, respectively. The Fermi level is set to zero. Figure 2: Partial DOS (PDOS) of (a) Zr in Graphene+Zr and (b) isolated Zr, respectively. The Fermi level is set to zero. Contribution of p-orbitals in both the cases is negligible. Figure 3: Geometrically optimized structures of (a)Graphene+Zr+9H2 and snapshot of molecular dynamics simulation of (b) Graphene+4Zr at 900K after 2.5 ps (c) Graphene+2Zr at 300 K after 2.5 ps and (d) Graphene+2Zr at 900K after 2.5 ps, respectively. Here, big bluish green spheres, pink spheres and small red spheres correspond to Zr atoms, C atoms and H atoms, respectively. Figure 4: Variation of difference in energy (diffusion energy barrier) with respect to initial configuration energy as a function of displacement of the Zr atom as it move from centre of one hexagon to the centre of adjacent hexagon. Figure 5: PDOS of sub-orbitalsdxy, dyz, dz2, dxz and dx2-y2of the 4d atomic orbital of (a) isolated Zr, (b) Zr in Graphene+Zr and (c) Zr in Graphene+Zr+1H2. The Fermi level is set to zero. The spin up and spin down states are presented in black and red, respectively. ACS Paragon Plus Environment


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Figure 6: PDOS plot of sub-orbitalspx, py, pzof the 2p state of (a) C of pristine graphene, (b) Non-bonded C in Graphene+Zr, (d) bonded C in Graphene+Zr and dxy, dyz, dx2, dxz and dx2-y2 of 4d state of (c) Zr in Graphene+Zr. The Fermi level is set to zero. Figure 7: PDOS of (a) Zr atom in Graphene+Zr (b) Zr atom in Graphene+Zr+1H2 and 1s of (c) H atom in isolated H2 molecule (d) H atom in Graphene+Zr+1H2.The Fermi level is set to zero. Figure8: PDOS of 5s and 4d states of Zr atom in (a) Graphene+Zr (b) Graphene+Zr+3H2 (c) Graphene+Zr+4H2.The Fermi level is set to zero. Contribution from p can be neglected. The Fermi level is set to zero. Figure 9: Isosurface for states just below the Fermi level for (a) Graphene+Zr, indicating Zr dx2-y2and dz2and C pz orbital hybridization(b) Graphene+Zr+1H2, indicating H 1s and Zr dz2orbital hybridization (c) Graphene+Zr+5H2 for top H2 molecule, indicating top H 1s and dxz orbital hybridization

(d)

Graphene+Zr+5H2for side H2 molecule, indicating side H 1s and dxy orbital hybridization with iso-value 1.5e. Here green, purple, and red spheres indicates Zr atom, C atom and H atom respectively. Figure10: PDOS ofsub-orbitals dxy,dyz,dz2, dxzand dx2-

y

2

of Zr 4d states present in (a) H 1s

ofGraphene+Zr+1H2(b) H 1s attached from top of Zr in Graphene+Zr+5H2 (c) H 1s attached from side of Zr in Graphene+Zr+5H2 Figure 11: Real-space plot of the effective spin density obtained by taking the difference between the charge densities of majority and minority spins (∆ρ =ρ↑ - ρ↓ ) for iso-value 0.1e for Graphene+Zr (upper panel), Graphene+Zr+1H2 (middle panel) and Graphene+Zr+3H2 (lower panel). Figure 12: Variation of desorption temperature as a function of magnetic moment. Blue curve represents fitted curve and red line corresponds to the simulated value.

TABLE CAPTIONS Table 1: The magnetic moment, average binding energy per H2 molecule and the corresponding desorption temperature for various configurations computed using GGA.


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40 Spin up Spin down

30

(a)

µ = 3.0µΒ

20

Density of states/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.41Å 1.43Å

B.E. = 2.4eV

10 0 -10

Graphene+Zr

40

(b)

Spin up Spin down

30 20

µ = 0µΒ

10

1.41Å

Ef

0 -10

Graphene -4

-3

-2

-1

0

1

2

3

4

E-Ef (eV) Figure 1: Total DOS of (a) Zr-decorated graphene and (b) pristine graphene, along with their

geometrically optimized structures, respectively. Here, pink spheres and bluish green spheres correspond to C and Zr atoms, respectively. The Fermi level is set to zero.

1



10 Zr s 8 Zr p 6 Zr d 4 2 0 -2 Ef -4 -6 -8 Zr in Graphene+Zr 100 -10 Zr s Zr p 50 Zr d

Partial density of states/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a)

(b)

0

µ = 2.0

-50

Isolated Zr -100 -6

-4

-2

0

2

4

6

E-Ef (eV)

Figure 2: Partial DOS (PDOS) of (a) Zr in Graphene+Zr and (b) isolated Zr, respectively. The Fermi level is set to zero. Contribution of p-orbitals in both the cases is negligible


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Figure 3: Geometrically optimized structures of (a)Graphene+Zr+9H2 and snapshot of molecular dynamics simulation of (b) Graphene+4Zr at 900K after 2.5 ps (c) Graphene+2Zr at 300 K after 2.5 ps and (d) Graphene+2Zr at 900K after 2.5 ps, respectively. Here, big bluish green spheres, pink spheres and small red spheres correspond to Zr atoms, C atoms and H atoms, respectively.

1



1.2 1.0

(E-Ein) eV

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0.8 0.6 0.4 0.2 0.0

-0.2 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Displacement (Å)

Figure 4: Variation of difference in energy (diffusion energy barrier) with respect to initial configuration energy as a function of displacement of the Zr atom as It move from centre of one hexagon to the centre of adjacent hexagon.


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40 20 dxy 0 -20 -40 40 20 dyz 0 Ef -20 -40 40 20 dz2 0 -20 -40 40 20 dxz 0 -20 -40 40 20 dx2-y2 0 -20 -40 -1.0 -0.5 0.0

Isolated Zr

0.5

(c)

(b)

(a)

1.0

E-Ef (eV)

1.5

12 8 4 0

dxy

12 8 4 0

dyz Ef

12 8 4 0

dz

12 8 4 0

dxz

Zr of Graphene+Zr

dxy

12 8 4 0

dyz E f

12 8 4 0

2

12 8 dx2 - y2 4 0 -4 2.0 -1.0 -0.5

12 8 4 0



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 8 4 0

0.0

0.5

1.0

1.5

2.0

E-Ef (eV)

Zr of Graphene+Zr+1H2

dz

2

dxz

12 8 dx2-y2 4 0 -4 -1.0 -0.5

0.0

0.5

E-Ef (eV)

Figure 5: PDOS of sub-orbitals dxy, dyz, dz2, dxz and dx2-y2 of the 4d atomic orbital of (a) isolated Zr, (b) Zr in Graphene+Zr and (c) Zr in Graphene+Zr+1H2. The Fermi level is set to zero. The spin up and spin down states are presented in black and red, respectively.

1 ACS Paragon Plus Environment

1.0

1.5

2.0


0.25

8

(a)

Ef

0.20

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(c)

Ef

0.15 4

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0.00

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-0.15

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-0.20

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C px

Zr dxy Zr dyz Zr dz Zr dxz Zr dx -y

-2

2

-4

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-0.25 0.25

(b)

0.20


0

-0.05


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0.15

2

Zr in Graphene+Zr

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Nonbonded C in Graphene+Zr

C px

-0.25 -4

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0

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C pz

-4

-3

E-Ef (eV)

-2

-1

0

1

2

3

4

E-Ef (eV)

Figure 6: PDOS plot of sub-orbitals px, py, pz of the 2p state of (a) C of pristine graphene, (b) Non-bonded C in Graphene+Zr, (d) bonded C in Graphene+Zr and dxy, dyz, dx2, dxz and dx2-y2 of 4d state of (c) Zr in Graphene+Zr. The Fermi level is set to zero.

1


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Zr s Zr p Zr d

8

Zr in Graphene+Zr


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Ef

6 4 2 0 -2 -4

(a)

-6 -4

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-2

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0.000 -0.005 -0.010

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8

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6 4 2 0 -2 -4

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Zr s Zr p Zr d

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-4



12


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

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Ef

1 0 -1

(d)

-2 -8

-6

-4

-2

0

2

4

6

8

E - Ef (eV)

E - Ef (eV)

Figure 7: PDOS of (a) Zr atom in Graphene+Zr (b) Zr atom in Graphene+Zr+1H2 and 1s of (c) H atom in isolated H2 molecule (d) H atom in Graphene+Zr+1H2.The Fermi level is set to zero.

1



15

Graphene+Zr

Zr s Zr d

10

(a)

5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ef

-5 15 10

Graphene+Zr+3H2

Zr s Zr d

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(b)

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Zr s Zr d

5

(c)

0 -5 -3

-2

-1

0

1

2

3

4

E-Ef (eV)

Figure 8: PDOS of 5s and 4d states of Zr atom in (a) Graphene+Zr (b) Graphene+Zr+3H2 (c) Graphene+Zr+4H2. The Fermi level is set to zero. Contribution from p can be neglected. The Fermi level is set to zero.

1


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Figure 9: Isosurface for states just below the Fermi level for (a) Graphene+Zr, indicating Zr dx2-y2 and dz2 and C pz orbital hybridization(b) Graphene+Zr+1H2, indicating H 1s and Zr dz2 orbital hybridization (c) Graphene+Zr+5H2 for top H2 molecule, indicating top H 1s and dxz orbital hybridization (d) Graphene+Zr+5H2 for side H2 molecule, indicating side H 1s and dxy orbital hybridization with iso-value 1.5e. Here green, purple, and red spheres indicates Zr atom, C atom and H atom respectively.

1



Hs

Ef

Zr dxy Zr dyz Zr dz

2

Zr dxz Zr dx -y 2

-8

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3.0 1.5 0.0 -1.5 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16

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Ef Top H s

Zr dxy Zr dyz Zr dz

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Zr dxz Zr dx -y 2

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-6

2

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(c)

3.0 1.5 0.0 -1.5 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16 8 0 -8 -16

Ef Side H s


8 4 0 12 8 4 0 12 8 4 0 12 8 4 0 12 8 4 0

(a)


3.0 1.5 0.0 -1.5 12


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-2

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Zr dxy Zr dyz Zr dz

2

Zr dxz Zr dx -y 2

-8

-6

2

-4

-2

0

2

E-Ef (eV)

Figure 10: PDOS of sub-orbitals dxy, dyz, dz2, dxz and dx2- y2 of Zr 4d states present in (a) H 1s of Graphene+Zr+1H2 (b) H 1s attached from top of Zr in Graphene+Zr+5H2 (c) H 1s attached from side of Zr in Graphene+Zr+5H2

1


4

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µ= 3.0µ B

µ= 2.0µ B

µ= 1.3µ B

Figure 11: Real-space plot of the effective spin density obtained by taking the difference between the charge densities of majority and minority spins (∆ρ =ρ↑ - ρ↓ ) for iso-value 0.1e for Graphene+Zr (upper panel), Graphene+Zr+1H2 (middle panel) and Graphene+Zr+3H2 (lower panel).

1



1100

Fitted Curve Calculated values

1000 900 800

TD (K)

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700

Fitted equation TD = T0+ aµb

600

T0 = 399 K

500

a = 302.38J-1TK b = 0.5

400 0

1

2

3

4

5

Magnetic moment βµB) Figure 12: Variation of desorption temperature as a function of magnetic moment. Blue curve represents fitted curve and red line corresponds to the simulated value.

1


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Table 1: The magnetic moment, average binding energy per H2 molecule and the corresponding desorption temperature for various configurations computed using GGA.

System

Magnetic

Binding energy (eV)

Desorption temperature (K)

moment (µ B) Graphene+Zr

3.0

2.4

Graphene+Zr+1H2

2.0

0.66

841

Graphene+Zr+5H2

0.42

0.51

637

Graphene+Zr+9H2

0.30

0.34

433

1



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Magnetic Moment Controlling Desorption Temperature in Hydrogen

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