Synergetic Effects toward Catalysis and Confinement of Magnesium

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Synergetic Effects Towards Catalysis and Confinement of Magnesium Hydride on Modified Graphene: A First-Principles Study Jian Zhang, Guanglin Xia, Zaiping Guo, and Dianwu Zhou J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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The Journal of Physical Chemistry

Synergetic Effects towards Catalysis and Confinement of Magnesium Hydride on Modified Graphene: A First-Principles Study Jian Zhang a, , Guanglin Xia b, Zaiping Guo*b, and Dianwu Zhou c a

Hunan Provincial Key Laboratory of Safety Design and Reliability Technology for Engineering

Vehicle, Changsha University of Science and Technology, Changsha 410114, China b

Institute for Superconducting and Electronic Materials, University of Wollongong, North

Wollongong, NSW 2522, Australia c

State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China

ABSTRACT: Graphene nanosheet has recently demonstrated catalytic and agglomeration blocking effects towards MgH2 nanoparticles. Nevertheless, there is a very limited understanding of the relationship between the structural characteristics of graphene nanosheet and the hydrogen sorption properties of MgH2 nanoparticles. Using first-principles calculations, we investigate the structural, energetic, and electronic properties of MgH2 clusters supported on pristine and modified graphene with carbon vacancy or heteroatom (B, N, Si, P, S, Fe, Co, Ni, and Al) doping. The results show that the formation ability of vacancy and heteroatom defects in the graphene lattice is enhanced in the order of vacancy, Al, Ni, S, Co, Fe, Si, P, B, and N. Among them, the B- and P-doped graphene nanosheets, especially the B-doped one, exhibit remarkable synergetic effects towards enhancing the catalysis and confinement of MgH2 hydride. Analysis of electronic structures shows that the direct bonding between MgH2 clusters and B/P-doped graphene, and the electron transfer 1

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from MgH2 clusters into the B/P-doped graphene are most likely to be the underlying reasons for the improved dispersion and enhanced dehydrogenation properties of MgH2 clusters.

1. INTRODUCTION Increasing fossil fuel depletion and environmental pollution have stimulated the search for alternative and sustainable forms of clean energy. Hydrogen is considered as an ideal energy carrier due to its high energy density, environmental friendliness, and renewability. Safe, efficient, and affordable hydrogen storage is the key to the success of any future hydrogen economy.1 Magnesium hydride, MgH2, has attracted extensive attention due to its great potential in hydrogen storage,2 thermal energy storage,3 as the negative electrode for lithium ion cells,4 and in optical hydrogen sensing applications.5 In particular, as a solid-state hydrogen storage medium, MgH2 has many advantages over other metal hydrides due to its high gravimetric (~7.6 wt%) and volumetric (~110 g/L) hydrogen storage capacities, the abundant natural resources of Mg, and its low cost.6 Nevertheless, the stubborn thermodynamic stability (dehydrogenation enthalpy, ∆H = -75 kJ/mol·H2) and sluggish kinetics of MgH2 result in its high temperature (~573 K) and low rate for hydrogen de/absorption, which constitute the major obstacles to its large-scale application.7 Over the last several decades, great efforts have been devoted to improving the thermodynamics and kinetics of MgH2. For example, nanostructuring,8,9 chemical alloying,10,11 adding catalysts,12,13 forming composites,14,15 etc. have been proved to be efficient modification strategies. Currently, the issues related to the sluggish sorption 2


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kinetics of MgH2 have already been solved. MgH2 can realize de-/rehydrogenation within several minutes under the synergistic effects of nanostructuring and catalysts.16,17 Achieving any significant change in the thermodynamics of MgH2 is still a big challenge, however. Although some experimental and theoretical studies have demonstrated that decreasing the particle size is capable of thermodynamically destabilizing MgH2 hydride, a small particle size, less than 2 nm, is needed to destabilize the strong Mg-H bonds to realize low temperature dehydrogenation.18 The small MgH2 nanoparticles often experience sintering and aggregation, however, due to their high surface energies, so that maintaining the stability of the nanostructure during de-/rehydrogenation is rather difficult. Although this issue could be solved to some extent by loading MgH2 into porous scaffolds, the dead weight of the scaffolds often leads to a significant loss of hydrogen storage capacity.19,20 In recent years, graphene (G), which has the form of a two-dimensional (2D) monolayer of sp2 carbon atoms arranged in a hexagonal network, has attracted tremendous attention in the energy-related catalytic area because of its light weight, large surface area, excellent thermal conductivity, and high chemical and mechanical stability.21,22 It has also been explored as a suitable substrate for anchoring well-dispersed MgH2 or catalyst nanoparticles.23-30 The ultrathin and flexible G not only exhibits efficient catalytic effects towards the hydrogen sorption properties of MgH2, but also effectively inhibits the agglomeration and growth of MgH2 during de-/rehydrogenation by interfacial confinement effects. For example, Liu et al. milled MgH2

with

5wt%

graphene

nanosheets

(GNs)

to

3


synthesize

MgH2-GN


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nanocomposites.23,24 It was shown that these smaller GNs on the composite surface not only provided more edge sites and hydrogen diffusion channels, but also prevented the nanograins from sintering and agglomerating, which led to enhancement of the hydrogen sorption properties of MgH2. Xia et al. successfully prepared GN supported ultrafine MgH2 nanoparticles with a homogeneous distribution and high loading percent via solvothermal25 and solvent-free methods,26 respectively. It was shown that the as-prepared MgH2-GN nanocomposites exhibited excellent dehydrogenation properties and cycling stability. They attributed the outstanding hydrogen storage performance to the nanosize effect of MgH2 nanoparticles, the stubborn interaction between the GNs and the MgH2 nanoparticles, and the dispersant effect and agglomeration resistance of the graphene substrate. This view was also confirmed through molecular dynamics (MD) simulations of the adsorption of MgH2 molecules on graphene nanoflakes.29 J. Zhang et al. also investigated the influences and mechanisms of G relating to the dehydrogenation properties of MgH2 based on experimental and first-principles studies.30 It was found that the G played a vital catalytic role towards improving the dehydrogenation properties of MgH2. Meanwhile, it also effectively inhibited the agglomeration of MgH2 particles during ball milling. First-principles calculations suggested that the enhanced dehydrogenation properties of MgH2-G composites could be ascribed to the reduced dehydrogenation enthalpy and dehydrogenation activation energy of MgH2 due to the catalytic role of G. Although previous studies have demonstrated the catalysis and agglomeration 4


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blocking (i.e. confinement) effects of graphene on MgH2, the graphene substrates or additives employed in previous work mainly involved pristine graphene. Few studies on the effects of modified graphenes, such as graphene with carbon vacancy or heteroatom doping, on MgH2 have been reported. In particular, there is a very limited understanding of the relationship between the structural characteristics of graphene nanosheet and the hydrogen sorption properties of MgH2 nanoparticles. In this work, the structural, energetic, and electronic properties of MgH2 clusters supported on pristine and modified graphene with carbon vacancy or heteroatom (B, N, Si, P, S, Fe, Co, Ni, and Al) doping are systematically investigated using first-principles calculations. The aim is to reveal the influences and mechanisms of modified graphene on the hydrogen sorption properties of MgH2 nanoparticles. The obtained results are expected to provide guidance for designing modified forms of graphene with more notable catalytic effects and agglomeration blocking capabilities for improved hydrogen storage in magnesium and other metal hydrides.

2. COMPUTATIONAL METHODOLOGY All the calculations in this work were performed with the DMol3 software package based on the dispersion-corrected density functional theory (DFT-D).31 The generalized gradient approximation (GGA)32 in Perdew-Wang 91 (PW91) format was adopted as the exchange-correction functional.33 All-electron Kohn-Sham wave functions were expanded in a double numeric plus polarization (DNP) basis.34 Sampling of the irreducible wedge of the Brillouin zone was performed with a regular Monkhorst-Pack grid of special k-points.35 All calculation models were relaxed to 5



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obtain the final structures with minimum total energy. The convergence criteria for relaxation were 2.0 × 10−5 Ha, 0.004 Ha/Å, and 0.005 Å for energy, gradient, and atomic displacement, respectively. To evaluate the dehydrogenation kinetics of MgH2, the transition states of H2 recombination and desorption from MgH2 were calculated by adopting linear synchronous transit / quadratic synchronous transit (LST/QST) tools,36 and the smearing energy was set at 0.005 Ha to realize rapid energy convergence. Figure 1a shows the crystal cell model of bulk MgH2 with the tetragonal rutile structure and space group P42/mnm.37 In order to simulate the MgH2/Mg nanoparticles, (MgH2/Mg)n (n = 1-6) clusters with different sizes were constructed,38 as shown in Figure S1 in the Supporting Information. To examine the most stable configurations of these clusters, some candidate (MgH2/Mg)n structures were considered. A 4×4 supercell (32 carbon atoms) was considered for the pristine graphene with a 20 Å vacuum layer thickness, as shown in Figure 1b. Single carbon vacancy and heteroatom (B-, N-, Si-, P-, S-, Fe-, Co-, Ni-, and Al-) doped graphenes were considered for the modified graphenes by removing or replacing one carbon atom in the pristine graphene, as shown in Figure 1c and Figure 1d, respectively. To examine the accuracy of computational parameters selected in this work, some test calculations on the structural parameters and energy of bulk MgH2, solid Mg, and gaseous H2 molecule were performed. The results are listed in Table 1. It can be seen that the calculated equilibrium lattice constants (a, c) of MgH2 and Mg are in good agreement with their experimental data.37,39 Moreover, the calculated H-H bond 6


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length (dH-H), and binding energy (Eb) of the H2 molecule are 0.749 Å and -4.509 eV, respectively, which are also close to the experimental values (0.741 Å and -4.48 eV).40 As a result, the computational parameters were employed in the subsequent calculations for determining the structural, energetic, and electronic properties of MgH2 clusters supported on pristine and modified graphene.

3. RESULTS AND DISCUSSION 3.1. Minimum-Energy Configurations and Dehydrogenation Properties of Pristine (MgH2/Mg)n Clusters. To examine the most stable configurations of (MgH2/Mg)n (n = 1-6) clusters, their candidate structures were fully relaxed, and the obtained configurations, as well as the absolute difference values for the total energies for (MgH2/Mg)n clusters, are given in Figure 2. It is clear that these candidate (MgH2/Mg)n clusters exhibit significantly different stabilities due to the obvious differences in total energy. For MgH2, Mg, and Mg2, there is only one geometric configuration because of the fewer atoms in these clusters. For (MgH2)2, a flat cluster configuration composed of two slightly bent MgH2 monomers forming a square is preferred for its relatively lower total energy. It is possible to generate larger linear cluster structures using the stable (MgH2)2 structure,38 but they are higher in total energy than the more compact configurations, as shown in Figure 2a. Notably, (MgH2)2, (MgH2)3, (MgH2)4, and (MgH2)6 clusters have high degrees of symmetry and clear structural similarities, with two 1-fold hydrogens at either end of the cluster. (MgH2)5, however, has only one 1-fold hydrogen. Additionally, (Mg)3, (Mg)4, (Mg)5, and (Mg)6 clusters also prefer to form more compact configurations, as shown in 7



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Figure 2b. Using these minimum-energy configurations, the dehydrogenation enthalpies, ∆Hd, of pristine (MgH2)n clusters for the reactions (MgH2)n→(Mg)n + nH2 were calculated using Eq. (1),41 ∆H d = ( Etot ( Mg )n + nEtot ( H 2 ) − Etot ( MgH 2 )n ) / n

(1)

where Etot ( MgH 2 ) n and Etot ( Mg ) n are the total energies of pristine (MgH2)n and (Mg)n clusters, respectively. Etot ( H 2 ) is the total energy of the gaseous H2 molecule. The calculated results are plotted in Figure 3. It can be seen that the MgH2 monomer has the smallest dehydrogenation enthalpy of 3.505 kJ/mol·H2 due to the size effect. With increasing cluster size, the enthalpy value is gradually increased. When n = 6, the dehydrogenation enthalpy is 96.691 kJ/mol·H2, which is very close to the calculated enthalpy value of 96.104 kJ/mol·H2 for bulk MgH2 in this work. It is notable that the size of the Mg6H12 cluster is only about 0.90 nm, which suggests that nanostructuring will not always improve the thermodynamics for H2 release, unless more (MgH2)5 or smaller clusters are synthesized. These results are consistent with those obtained by Wagemanns et al.42 and Shevlin et al..38 Wagemanns et al.42 found that nanostructuring MgH2 improved the dehydrogenation thermodynamics when n = 2-6, while Shevlin et al.38 found that only the (MgH2)2 cluster had a smaller dehydrogenation enthalpy than the bulk MgH2. The difference in cluster size between our and other studies can be attributed to the different choices of calculation functionals and cluster configurations. 3.2. Synergetic Effects of Catalysis and Confinement by Modified Graphene 8


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on MgH2. The formation ability of modified graphenes was first examined by calculating the formation enthalpies 43 of single carbon vacancy and heteratom X (X = B, N, Si, P, S, Fe, Co, Ni, and Al) in the graphene lattice. It can be seen from Figure S2 that the formation ability of vacancy and heteroatom defects in the graphene lattice is enhanced in the order of vacancy, Al, Ni, S, Co, Fe, Si, P, B, and N. Moreover, the relaxed modified graphenes still maintain the planar structures of pristine graphene, as shown in Figure S3, suggesting their high structural stabilities. Then, the influence of modified graphene on the dehydrogenation properties of MgH2 was evaluated by calculating the dehydrogenation enthalpies ∆H d of (MgH2)n clusters supported on modified graphene using Eqs. (2) and (3), respectively,41 ∆H d = ( Etot ((Mg ) n + G(C − vacancy)) + nEtot ( H 2 ) − Etot ((MgH 2 ) n + G(C − vacancy))) / n (2) ∆H d = ( Etot (( Mg ) n + G ( X − doped )) + nEtot ( H 2 ) − Etot (( MgH 2 ) n + G ( X − doped ))) / n

where

Etot ((MgH 2 ) n + G(C − vacancy)) and

Etot (( MgH 2 ) n + G( X − doped )) are

the

(3) total

energies of (MgH2)n clusters supported on carbon vacancy and heteroatom X-doped graphene, respectively. Etot ((Mg ) n + G(C − vacancy)) and Etot (( Mg ) n + G ( X − doped )) are the total energies of (Mg)n clusters supported on carbon vacancy and heteroatom X-doped graphene, respectively. Here, we chose the Mg4H8/Mg4 and Mg6H12/Mg6 clusters as prototypes and performed structural optimizations and calculations of dehydrogenation enthalpies. The optimized configurations of Mg4H8/Mg4 clusters in different systems are presented in Figure 4. It can be seen that the Mg4H8 and Mg4 clusters are structurally distorted in varying degrees on pristine and modified graphene substrates. In particular, the Mg4 cluster in most systems has a tendency to 9



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transform to the planar structure, which is quite different from the pristine Mg4 cluster with a tetrahedral configuration. Meanwhile, these graphene substrates, except for the pristine and N-doped ones, also exhibit local structural distortion near the vacancy or doped atoms. Most doped heteroatoms extend out of the graphene plane and pull the Mg4H8/Mg4 clusters closer. Similar cases can also be seen in the optimized configurations of Mg6H12/Mg6 clusters in different systems, as shown in Figure S4. Evidently, there is an interfacial coupling effect between MgH2/Mg and modified graphene, which will certainly alter the dehydrogenation properties of MgH2. Using these relaxed structures, the dehydrogenation enthalpies of the Mg4H8 and Mg6H12 clusters in different systems were calculated, and the obtained results are plotted in Figure 5. It can be seen that these modified forms of graphene have a significant influence on the dehydrogenation thermodynamics of MgH2. For Mg4H8 clusters, as shown in Figure 5a, the B-, N-, Si-, and P-doped graphene nanosheets play catalytic roles in promoting the dehydrogenation of the Mg4H8 cluster, while the vacancy, S-, Fe-, Co-, Ni-, and Al-doped graphene nanosheets exhibit negative effects in different degrees. For the larger Mg6H12 cluster, as shown in Figure 5b, the B-, Si-, P-, S-, Ni-, and Al-doped graphene nanosheets play catalytic roles in promoting the dehydrogenation of the Mg6H12 cluster, and the vacancy, N-, Fe-, and Co-doped graphene nanosheets present negative effects. The differences in catalytic effects may have originated from the influences of cluster size on the dehydrogenation enthalpy of MgH2. Notably, the catalytic roles of B- and P-doped graphene nanosheets either on Mg4H8 or on Mg6H12 clusters are more remarkable relative to the other modified 10


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graphenes. He et al.44 demonstrated that P-doped ordered mesoporous carbon (CMK-3) scaffold could significantly enhance the hydrogen release properties of nanoconfined MgH2 at low temperature, based on both theoretical and experimental investigations. Xu et al.45 found that B-doped graphene could cause instantaneous H2 release in supported NaAlH4 upon relaxation, using periodic density functional theory calculations. These previous studies confirm our calculated results to some extent. In order to further examine the agglomeration-blocking capability or confinement effects of modified graphene on MgH2 nanoparticles, the binding energies, Eb , between Mg4H8/Mg6H12 clusters and pristine/modified graphene substrates were calculated using Eqs. (4), (5), and (6), respectively46 Eb = Etot ( MgH 2 ) n + Etot (G) − Etot ((MgH 2 ) n + G)

(4)

Eb = Etot ( MgH 2 ) n + Etot (G(C − vacancy)) − Etot ((MgH 2 ) n + G(C − vacancy))

(5)

Eb = Etot ( MgH 2 ) n + Etot (G( X − doped )) − Etot ((MgH 2 ) n + G( X − doped))

(6)

The calculated results are plotted in Figure 6. It was found that the binding energies are all positive between Mg4H8/Mg6H12 clusters and pristine/modified graphene substrates. This reveals that these graphene substrates are conducive to restraining the agglomeration of MgH2 nanoparticles, which is consistent with previous experiments.23-30 It is notable that the modified graphene, except for the N-doped graphene, provides larger or similar binding energies relative to the pristine material, which suggests that carbon vacancies and most heteroatoms in the graphene lattice can enhance the ability to block agglomeration of MgH2 nanoparticles. Although Feand Co-doped graphene nanosheets provide larger binding energies, their catalytic 11



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effects towards the dehydrogenation of MgH2 are undesirable. Similar results are also achieved from the calculations of binding energies between Mg4/Mg6 clusters and graphene in different systems, as shown in Figure S5. So, based on a comprehensive point of view, including formation enthalpy, dehydrogenation enthalpy, and binding energy, the B-, and P-doped graphenes possess remarkable synergetic effects towards enhancing catalysis on and confinement of MgH2 nanoparticles. To explore whether the modified graphene influences the dehydrogenation kinetics of MgH2, we chose an Mg4H8 cluster supported on B/P-doped graphene (i.e. Mg4H8+G(B/P-doped)) as the prototype, and the transition states of H2 recombination and desorption from the Mg4H8 cluster were further calculated by adopting LST/QST tools.36 For comparison, the recombination and desorption of one 1-fold hydrogen and its nearest neighbor 2-fold hydrogen in the pristine Mg4H8, Mg4H8+G, Mg4H8+G(B-doped), and Mg4H8+G(P-doped) systems were also considered. The calculated dehydrogenation activation energies are plotted in Figure 7. In the case of the pristine Mg4H8 cluster, its activation energy was calculated as 184.636 kJ/mol, which is close to the experimental value of 156 kJ/mol47 and the calculated value of 172 kJ/mol.48 The slight difference in values is possibly due to the different physical models used to describe the rate determining processes. In the case of the Mg4H8+G system, the activation energy is reduced to 175.899 kJ/mol, so that the pristine graphene has a catalytic effect to some extent on the dehydrogenation kinetics of MgH2. As compared with pristine graphene, the B- and P-doped graphenes significantly decrease the dehydrogenation activation energy (40.785 kJ/mol and 12


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111.126 kJ/mol, respectively) of MgH2. Comparatively, the effect of B-doped graphene is more outstanding. Thus, it is further demonstrated that the B- or P-doped graphene, especially the B-doped one, is an ideal substrate or additive to synergistically improve the thermodynamics, kinetics, and aggregation-resistance of MgH2 nanoparticles. These results provide important guidance for optimizing graphene-based catalysts and enhancing the hydrogen sorption properties of magnesium hydride. 3.3. Electronic Structures. To illustrate the catalysis and confinement mechanisms of modified graphene that act on MgH2, the total and deformation charge densities of the pristine Mg4H8, Mg4H8+G, Mg4H8+G(B-doped), and Mg4H8+G(P-doped) systems were calculated and are presented in Figure 8. For the pristine Mg4H8 cluster, there is a partial overlapping of electron clouds around the Mg and H atoms, as seen from the total charge density plot. Meanwhile, the largest amounts of electron clouds are mainly gathered around the H atoms, as seen from the deformation charge density plot. This means that there are typical ionic-covalent bonding interactions between Mg and H within the pristine Mg4H8 cluster. For the Mg4H8+G system, there is no overlapping of electron clouds between the Mg4H8 cluster and the pristine graphene, which suggests the physical adsorption of Mg4H8 on the surface of the graphene. This is consistent with our previous experimental result that no MgH2(Mg)-C-containing phases were detected in a milled MgH2-G mixture.30 In addition, the weak physical adsorption interactions between the Mg4H8 cluster and the pristine graphene also account well for the relatively lower confinement effects of pristine graphene on 13



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MgH2, as shown in Figure 6. Furthermore, the bonding interactions between Mg and H within the Mg4H8 cluster are somewhat weakened due to slight structural distortion of the Mg4H8 cluster under the influence of pristine graphene, which leads to less overlapping of electron clouds between Mg and H relative to the pristine Mg4H8 cluster. In the cases of the Mg4H8+G(B-doped) and Mg4H8+G(P-doped) systems, serious structural distortion of the Mg4H8 cluster can be observed, and the distance between the Mg4H8 cluster and the graphene is shortened due to the confinement effects of the B- or P-doped graphene substrate. There is an obvious overlapping of electron clouds between the Mg4H8 cluster and the local B- or P-doped region in the graphene, suggesting electron transfer among the Mg4H8, the B or P atom, and the graphene. Additionally, the overlapping of electron clouds between Mg and H is significantly decreased, implying weakened Mg-H bonding interactions. From the deformation electron density plots, it can be seen that the ionic bonding interactions between Mg and H play the dominant role in the four systems. Table 2 presents the Mulliken charges of Mg and H, and the bond length of Mg-H in the four systems above. It can be found that the Mg and H in the four systems are positively and negatively charged, respectively. For the pristine Mg4H8 and Mg4H8+G systems, the absolute charges on Mg are just twice as many as that on H, which means that there is electric neutrality of the Mg4H8 cluster in the two systems. This is consistent with the results from total charge densities that there is no electron transfer between the Mg4H8 cluster and the neighboring environment. By comparison, the absolute values of the charges on Mg and H in Mg4H8+G are both larger than in 14


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pristine Mg4H8, suggesting that electrons are being transferred from Mg to H within the Mg4H8 cluster due to the addition of pristine graphene. Additionally, the average bond length of Mg-H in the Mg4H8+G system is slightly larger than that in the pristine Mg4H8 due to the slight structural distortion of the Mg4H8 cluster. In the case of the Mg4H8 +G(B-doped) and Mg4H8 +G(P-doped) systems, the average bond length between Mg and H is larger than in the former two systems because of the serious structural distortion of the Mg4H8 cluster under the influence of the B- or P-doped graphene substrate. More importantly, the Mg4H8 cluster in this system is no longer electrically neutral but positively charged, which indicates electron transfer from the Mg4H8 cluster into the B- or P-doped graphene substrate. Figure 9 presents the total and partial densities of states of the Mg4H8 +G, Mg4H8 +G(B-doped), and Mg4H8 +G(P-doped) systems. The Fermi level (EF) is set at zero and used as a reference. For the Mg4H8 cluster in the Mg4H8 +G system, as shown in Figure 9a, the valence band is mainly dominated by H (s) orbitals and the conduction band mainly originates from the contributions of Mg (s) and (p) orbitals, which means that there are strong ionic bonding interactions between Mg and H. Additionally, a few Mg (s) and Mg (p) orbitals can also be seen to be hybridized with H (s) orbitals in the valence band, which contributes some covalent bonding interactions between Mg and H. These results are consistent with those analyzed from the charge density plots above. In the cases of the Mg4H8 +G(B-doped) and Mg4H8 +G(P-doped) systems, as shown in Figure 9b and 9c, obvious hybridization of B/P (s)(p) and C (s)(p) orbitals can be seen, implying strong covalent bonding interactions between B/P and C atoms 15



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in graphene. As compared with the Mg4H8 +G system, the bonding peak heights for Mg and H atoms below EF and the anti-bonding peak heights for Mg above EF in the Mg4H8+G(B-doped) and Mg4H8+G(P-doped) systems are somewhat reduced, suggesting weakened ionic-covalent bonding interactions between Mg and H due to B or P doping in graphene. Thus, it can be deduced from the charge densities, Mulliken charges, and densities of states that the direct bonding between the MgH2 clusters and the modified graphene, the electron transfer from Mg4H8 clusters into the modified graphene, and the weakened ionic-covalent bonding interactions between Mg and H are most likely to be the underlying reasons for the improved dispersion and enhanced dehydrogenation properties of MgH2 clusters supported on modified graphene. These results shed new light on designing modified graphene with more notable catalytic effects and the ability to block agglomeration of magnesium and other metal hydrides for hydrogen storage.

4. CONCLUSIONS In this work, the structural, energetic, and electronic properties of MgH2 clusters supported on pristine graphene and graphene modified by carbon vacancy or heteroatom (B, N, Si, P, S, Fe, Co, Ni, and Al) doping were systematically investigated using first-principles calculations based on the dispersion-corrected density functional theory. The calculations of formation enthalpy show that the formation ability of carbon vacancy and various heteroatom defects in the graphene lattice is gradually enhanced in the order of carbon vacancy, Al, Ni, S, Co, Fe, Si, P, B, and N. The calculations of dehydrogenation enthalpy and binding energy reveal that 16


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the B and P-doped graphenes, especially the B-doped one, present remarkable synergetic effects towards enhancing the catalysis of and confinement on MgH2 nanoparticles. Analysis of the electronic structures shows that the direct bonding between MgH2 clusters and modified graphene, the electron transfer from MgH2 clusters into the modified graphene, and the weakened ionic-covalent bonding interactions between Mg and H are most likely to be the underlying reasons for the improved dispersion and enhanced dehydrogenation properties of MgH2 clusters supported on modified graphene.

ASSOCIATED CONTENT Supporting Information Figure of calculation models of (MgH2/Mg)n (n = 1-6) clusters; Figure of calculated formation enthalpies of vacancy and heteratoms in graphene; Figure of optimized pristine and modified graphene; Figure of optimized Mg6H12 and Mg6 clusters in different systems; Figure of calculated binding energies between Mg4 (a) / Mg6 (b) clusters and graphene in different systems. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Z.P. Guo: e-mail, [email protected]. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS 17



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This work was supported by the National Natural Science Foundation of China (No. 51401036),

the

Hunan

Provincial

Natural

Science

Foundation

of

China

(No.17JJ2263), and the Science Research Project of Hunan Province Office of Education (No. 16K001). The authors would like to thank Dr. Tania Silver for the revision of the manuscript.

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Shevlin,

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24


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Table 1. Calculated equilibrium lattice constants (a, c), bond length (d), and binding energy (Eb) for bulk MgH2, solid Mg, and gaseous H2 molecule Materials

Parameter

Cal.

Exp. 37,39,40

MgH2

a (Å)

4.508

4.501

c (Å)

3.046

3.010

a (Å)

3.144

3.21

c (Å)

5.228

5.21

dH-H (Å)

0.749

0.741

Eb (eV)

-4.509

-4.48

Mg

H2

Table 2. Mulliken charges (∆Q) of Mg and H, and average bond length (dMg-H) of Mg-H in the pristine Mg4H8, Mg4H8+G, Mg4H8 +G(B-doped), and Mg4H8 +G(P-doped) systems System

∆QMg (e)

∆QH (e)

dMg-H (Å)

Mg4H8

0.664

-0.332

1.909

Mg4H8 +G

0.748

-0.374

2.081

Mg4H8 +G(B-doped)

0.780

-0.347

2.163

Mg4H8 +G(P-doped)

0.764

-0.361

2.087

25



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(a) Mg H C Heteroatoms

(b)

(c)

(d)

Figure 1. Calculation models of bulk MgH2 (a), pristine graphene (b), vacancy graphene (c), and heteroatom-doped graphene (d).

26


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(a)

(b)

Figure 2. Minimum-energy configurations of (MgH2)n (a) and (Mg)n (b) (n = 1-6) clusters.

27



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Figure 3. Calculated dehydrogenation enthalpies of pristine (MgH2)n (n = 1-6) clusters.

28


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Pristine Mg4H8/Mg4

Mg4H8/Mg4+G

Mg4H8/Mg4+G (C-vacancy)

Mg4H8/Mg4+G (B-doped)

Mg4H8/Mg4+G (N-doped)

Mg4H8/Mg4+G (Si-doped)

Mg4H8/Mg4+G (P-doped)

Mg4H8/Mg4+G (S-doped)

Mg4H8/Mg4+G (Fe-doped)

Mg4H8/Mg4+G (Co-doped)

Mg4H8/Mg4+G (Ni-doped)

Mg4H8/Mg4+G (Al-doped)

Figure 4. Optimized Mg4H8 and Mg4 clusters in different systems.

29



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(a)

(b)

Figure 5. Calculated dehydrogenation enthalpies of Mg4H8 (a) and Mg6H12 (b) clusters in different systems.

30


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(a)

(b)

Figure 6. Calculated binding energies between Mg4H8 (a) / Mg6H12 (b) clusters and graphene in different systems.

31



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Figure 7. Calculated dehydrogenation activation energies of the pristine Mg4H8, Mg4H8+G, Mg4H8+G(B-doped), and Mg4H8+G(P-doped) systems.

32


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Total electron density (Isovalue=0.2)

Deformation electron density (Isovalue=0.05)

Mg4H8

Mg4H8 +G

Mg4H8 +G (B-doped)

Mg4H8 +G (P-doped)

Figure 8. Total and deformation charge densities of the pristine Mg4H8, Mg4H8+G, Mg4H8+G(B-doped), and Mg4H8+G(P-doped) systems.

33



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(a)

(b)

(c)

Figure 9. Total and partial densities of states of the Mg4H8+G (a), Mg4H8+G(B-doped) (b), and Mg4H8+G(P-doped) (c) systems. 34


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TOC Graphic

Dehydrogenation

35


Synergetic Effects toward Catalysis and Confinement of Magnesium

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