First-principles insight into hydrogen adsorption over Co4 anchored on defective graphene

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First-principles insight into hydrogen adsorption over Co4 anchored on defective graphene ⁎

Shuhong Maa, , Juncai Chenb, Lifang Wanga, Zhaoyong Jiaoa, a b

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School of Physics, Henan Normal University, Xinxiang, Henan 453007, China School of Materials Science and Engineering, Henan Institute of Technology, Xinxiang 453003, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrogen Storage Co4 cluster Defective graphene First-principles calculation

The interaction of hydrogen with graphene-supported Co4 was studied by first-principles calculation, with a particular focus on the role of support defects in modulating the adsorption properties of hydrogen. It is shown that point defects in graphene can effectively improve the capacity of Co4 for hydrogen storage by modifying the interactions between metal cluster and supports, but exert different effects. By virtue of excellent structural stability and favorable hydrogen uptake, Co4 decorated on defective graphene with dopant N accompanying with C-vacancy stand out as promising hydrogen storage media, and such a better performance is more likely relating to a lower d-band center of supported-Co4. This work explores the possibility of Co4-decorated defective graphene sheets as hydrogen storage materials through introducing the appropriate defects in support.

1. Introduction The clean and renewable hydrogen energy is deemed as an optimal candidate to substitute the traditional fuels, because of the advantages of high efficiency, abundance and high gravimetric density [1]. Presently, a great tricky challenge is to exploit desirable storage materials with high hydrogen uptake, reversible hydrogen adsorption and desorption at ambient conditions [2]. In the past decades, the relatively light-weight carbon-based nanostructures have been attempted to hydrogen storage, including carbon nanotubes [3–5], grapheme [6] and so on. However, their quite weak hydrogen binding energy and fairly low hydrogen storage capacity at ambient conditions [7–9], are far below the expectations proposed by the U.S. Department of Energy (DOE). Thereupon, great endeavors were devoted to explore desirable hydrogen storage materials both theoretically and experimentally [1,10–18], and noteworthily surface functionalizing with metal atoms proposed as one strategy [9,19], can facilitate reversible hydrogen adsorption, enhance high hydrogen capacity simultaneously, resultantly improving the hydrogen storage performance of carbon-based materials [20–23]. This had been addressed in the investigations on alkali and alkali-earth atoms [23–25], transition metal nanoparticles i.e. V4 [26], Ni13 [27], Pdn (n = 2–6) [13,28,29], Ti4 and Pt4 [30] decorated on graphene nanosheets. Meanwhile, it was well documented that defects existed unavoidably in graphene sheet, i.e. carbon-

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vacancies and heteroatom impurities, can efficiently stabilize metal atoms and achieve outstanding capability for hydrogen storage applications, by regulating the interactions between metals and support [23,25,31,32]. Recently, the experimental studies of Jung [33] and Chen [34] groups demonstrated that the successfully prepared graphene-cobalt nanocomposite exhibits higher hydrogen storage uptake and better stability at room temperature than their single components, which could be promising medium for electrochemical hydrogen storage. So far, only a few theoretical publications reported the adsorption of hydrogen on free-standing cobalt clusters [14,35,36] and the properties of small Co clusters supported on grapheme [37–40], based on density functional theory calculations. For instance, Pour et al. [36] concluded that the interaction strength between hydrogen atom and Co cluster enhances with increasing of the number of atoms in cluster Con (n = 4–24), and Buendía et al. [14] found the adsorption of H2 exclusively dissociative on the neutral clusters Con (n = 1–5). A recent computational study by López et al. [35] investigated the mechanisms for adsorption and dissociation of molecular hydrogen on clusters Co6 and Co13, and predicted a promising 8.4% content of hydrogen by weight for hydrogen storage over Co6. So, it is intriguing how graphene support affects the interaction between hydrogen and Co cluster and defects in support modulate the hydrogen storage performance, which is crucial to comprehend the insight of hydrogen storage. Herein, by performing first-principles calculation we studied the

Corresponding authors. E-mail addresses: [email protected] (S. Ma), [email protected] (Z. Jiao).

https://doi.org/10.1016/j.apsusc.2019.144413 Received 5 August 2019; Received in revised form 5 October 2019; Accepted 14 October 2019 Available online 26 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Shuhong Ma, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144413

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Fig. 1. (a–f) Optimized geometries for one and two H2 molecules adsorption and the energy profiles for their dissociation on Co4, and (g–j) for the hydrogen adsorbing configurations Co4(H2)n(H)4 (n = 2, 4, 6, 8) together with the sequential binding energy in eV. (Color map: blue–cobalt, pink–hydrogen, the values more/ less than 1.00 denote the bond lengths (in Å) of CoeH/HeH.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

convergence criterion for energy was chosen to be 10−6 eV. A denser 12 × 12 × 1 k-point mesh was used for electronic structure calculations. Additional calculations were conducted by the DFT-D2 [46] method to examine the van der Waals (vdW) dispersion effects on hydrogen adsorption energies. A nominal contribution less than 0.08 eV was observed from Table S1 in Supporting Information, and thus the present results do not include the vdW correction, similar to the prior studies [47,48]. The present results are converged with the parameters set in our calculations. The average adsorption energy (Eads ) of hydrogen over the freestanding or supported-Co4 was calculated by: Eads = (ECo4 + xE H2 − ExH2 @ Co4 )/ x , where x is the number of adsorbed H2, E H2 denotes the energy of free hydrogen molecule in vacuum, and ExH2 @ Co4 and ECo4 are the total energies of the systems with and without x hydrogen molecules adsorbed on free-standing or supported-Co4, respectively. According to the previous publication [48], the sequential H2 binding energy (ΔEx ) defined with ΔEx = E(x − 1) H2 @ Co4 + E H2 − ExH2 @ Co4 , was used to determine the saturation of hydrogen adsorption, and ΔEx > 0 indicates that an adsorption is energetically favorable. Noticeably, the energy of H2 molecule was used for all the cases of both molecular and dissociative H2 adsorbing. Bader charge analysis [49] was adopted to evaluate electron transfer, and the climbing image nudged elastic (CI-NEB) [50] method was utilized to estimate the activation energy for hydrogen migration

interaction of hydrogen with cluster Co4 decorated on pristine and defective graphene sheets, in view of the good stability of Co4 [41] anchored on graphene sheets with and without C-vacancies. The roles of various defective graphene supports in modulating the interaction of hydrogen with Co4 were discussed in more detail, including C-vacancy, heteroatom dopants (B, N, P) and their combinations. The findings highlight that Co4 anchored on defective graphene sheets with dopant N and C-vacancy outperform the others as promising materials for hydrogen storage. 2. Computational details Spin-polarized first-principles calculation was carried out with the projector-augmented wave (PAW) potentials [42] and Perdew-BurkeErnzerh (PBE) exchange-correlation functional within generalized gradient approximation (GGA) [43], as implemented in Vienna Ab-initio Simulation Package (VASP) [44], together with a kinetic energy cutoff of 450 eV. A 4 × 4 × 1 supercell was used to model single-layer graphene with the previously calculated lattice constant of 2.47 Å [41], and a 15 Å vacuum layer was set to avoid the interaction between the neighboring supercells. For geometrical optimization, Brillouin zone (BZ) integration was sampled using the Monkhorst-Pack [45] scheme with 7 × 7 × 1 k-point grid, and all the atoms were fully relaxed until the forces on each atom were less than 0.02 eV/Å, as well the 2

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shown to adsorb dissociatively and the most favorable configuration is that two H atoms locate at the adjacent CoeCo bridge sites (Co4(H)2) (Fig. 1b) as compared to the others (Fig. S2(d, e)). Moreover, the CINEB simulations in Fig. 1c also ascertain that dissociation of one H2 molecules is spontaneous and exothermic on free Co4. Similar cases were evidenced for H2 dissociative adsorption on Co6 [35], Co13 [35], and Pd4 [51] clusters. Furthermore, one can observe from Fig. 1d–f and Fig. S2f–k that two hydrogen molecules adsorption on Co4 still yields the favorably dissociative configuration of Co4(H)4, resulting in four H atoms strongly bound in the edge-bridge sites of Co4 (Fig. 1e), which exhibits structural transformation from tetrahedron to planar rhombus. Meanwhile, the spontaneously dissociative adsorption of two H2 molecules on Co4 is ascertained by the CI-NEB results in Fig. 1f. Subsequently, multiple hydrogen molecules were added successively starting with the favorable configuration Co4(H)4 until it reaches saturation with another eight H2 molecules adsorbed over the Co4(H)4 hydride, as after adding another one H2 molecule a very weakly physisorption is found with the sequential binding energy ΔEx close to zero (0.016 eV), together with a distance more than 3.0 Å between the additional H2 and the hydride (not shown). The results in Fig. 1g–j indicate that successive hydrogen molecules prefer to adsorb atop site of each Co atom in the molecular form rather than the dissociative one. With hydrogen molecules aggregating, the adsorption energy finally is lowered to 0.61 eV per H2 (see Fig. 2a) at the saturated Co4(H2)8(H)4 complex, including two dissociative and eight molecular hydrogen, adding up to the maximum number of ten. We note that the hydrogen uptake of Co4 is less that of Co6 [35], in good agreement with. Upon hydrogen aggregating, the elongated CoeCo bond and the nearest CoeH bond length oscillate slightly in the ranges of 2.20–2.37 Å and 1.65–1.70 Å as shown in Fig. 2a, respectively, while HeH bond is elongated in the range of 0.83–0.93 Å relative to free H2 (0.75 Å). The key parameters fit well with those of Kubas complex (dH–H = 0.8–0.9° Å and dCo–H = 1.65–2.05 Å) as proposed in the literature [47]. Moreover, the calculations (see the bottom panel of Fig. 2a) indicate that some electrons transfer from metallic cluster to adsorbates and the

and dissociation, and the transition state was confirmed with single imaginary frequency. 3. Results and discussion According to the publications [35,51], we first discussed the competition between molecular and dissociative adsorption of one and two hydrogen molecules to clarify the adsorption behavior on free Co4, then the hydrogen storage capacity was evaluated by adding successively multiple hydrogen molecules up to saturation. Next, the support of graphene was introduced to make clear how about the hydrogen adsorption strength and the uptake capacity, as well how the defects in graphene sheet modulate the hydrogen storage performance. Such defects were examined as carbon monovacancy; heteroatom dopants B, N, P; and their combinations i.e. doping B or N atom in proximity to a single C-vacancy (see Fig. S1), which are abbreviated as SVG; BVG, NVG, PVG; as well as B-SV, 3B-SV, N-SV, and 3 N-SV (pyridinic) realized experimentally [52], respectively. Afterwards, the underlying mechanism was elaborated based on electronic structures of adsorbed systems. Finally the hydrogen storage performance of Co4/graphene (denoted as Co4/GRA) was compared with other similar systems. 3.1. Adsorbing of hydrogen on free-standing Co4 Hydrogen adsorbed on top, bridge, and hollow sites of Co4 were examined to determine the lowest-energy configurations for molecular H2 molecule as well as its dissociative (two H atoms) adsorption. For simplicity, multiple hydrogen adsorbed on cluster Co4 in their molecular/dissociative forms were denoted as Co4(H2)M/(H)2N, with M (N) representing the number of hydrogen molecules. Present calculated results are shown in Figs. 1 and 2, including the optimized structures and key parameters, adsorption energies, charge transfer and electronic density of states (DOS). As displayed in Fig. 1a, single H2 adsorbs on-top Co atom in the distorted tetrahedron Co4 (denoted as Co4H2) with an adsorption energy of 0.64 eV. The initially placed H2 in bridge and hollow sites were

Fig. 2. (a) The numerical results for successive hydrogen adsorbed on Co4 varying with the number of H2, top to bottom: the average adsorption energy (Eads); bond lengths of CoeCo and CoeH, and the electrons (Δq) transferred from cluster Co4 to hydrogen; (b) projected density of states (DOS) on Co4-3d, atomic H and molecular H2 states for the configurations of Co4(H2)M(H)4 (M = 4, 8). (The green area in (a) stands for the reversible H2 adsorption/desorption energy range; Fermi energy is shifted to zero in (b, c); adsorption energy data for Co6 is cited from Ref. [35]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3

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Fig. 3. (a) The average adsorption energy (Eads) of H2 over Co4/PG varying with the number of adsorbed H2; (b)–(d) top and side views for the configurations of Co4/PG@(H2)M(H)4 (M = 0, 3, 5). (Color map: blue–cobalt, pink–hydrogen, gold–carbon). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Same as Fig. 3 but for H2 adsorbed over Co4/SVG.

3.2. Modulating hydrogen adsorption on Co4 by graphene supports

amount averaged on per H2 exhibits an increase-to-decrease trend with the aggregating of hydrogen, in good accordance with the strong-toweak hydrogen-metal interaction. Meanwhile, it is observed from the density of states (DOS) plots in Fig. 2b that several sharp DOS peaks of atomic H mainly appeared in the energy region of −4.0 to −6.0 eV hybridize with Co-3d states, showing a strong interaction between Co4 and H, while the molecular orbitals of H2 dominantly overlap with Co3d states, forming bonding and anti-bonding states in the wide energy range from −6.0 to 5.0 eV. We also notice a relatively slight upward shift for Co-3d orbitals of the spin-up states, leading to a reduction in magnetic moment from 10 μB for free Co4 to 6 μB from the saturated hydrogen adsorption case.

3.2.1. Over pristine graphene (PG) As reported in the previous study [41], Co4 adsorbed on pristine graphene (Co4/PG) favors in the tetragonal configuration with a binding energy of 1.30 eV/cluster. Upon hydrogen molecule adsorption, the first two H2 molecules (see Fig. 3b) still favorably adsorb dissociatively with atomic H residing in the adjacent CoeCo bridge and hollow sites over Co4/PG (denoted as Co4/PG@(H)4), and the PG support weakens the interaction between hydrogen and Co4, with the adsorption energy lowered by 0.45 eV/H2 relative to the free-standing case. Subsequently, multiple hydrogen molecules adsorb successively in 4

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Fig. 5. (a) The average adsorption energy (Eads) of H2 varying with its number on Co4 supported by B-, N- and P-doped graphene sheets; (b)–(d) top and side views for the configurations of Co4/BVG@(H2)M(H)4 (M = 0, 3, 5). (Color map: blue–cobalt, pink–hydrogen, gold–carbon, green–boron). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

adsorbed hydrogen to that of on PG and the similar optimized configurations. For instance, the first two hydrogen molecules also adsorb dissociatively with the average adsorption energy of 1.06 eV/H2, very close to that over Co4/PG (see Table S3). Meanwhile, there would be another six H2 molecules adsorbed at the saturated configuration of Co4/SVG@(H2)6(H)4 (Fig. 4d), with the adsorption energy of 0.53 eV/ H2. We notice that four hydrogen molecules could adsorb over Pd4/SVG with Eads = 0.50 eV/H2 [47]. Over Co4/SVG, the tetrahedron orientations of Co4 as well as the key parameters in Table S3, including the CoeCo, CoeH and HeH bond lengths are barely changed relative to that over Co4/PG. Overall, the presence of vacancy in graphene enhances the hydrogen storage capacity with faintly larger adsorption strength than that on PG.

Table 1 Calculated results for Co4 adsorbed on a series of graphene supports. Binding energy (Eb), d-band center (εd) of Co4, nearest CoeC distance (dCo–C), average CoeCo bond length (dCo–Co) and a loss of electrons (Δq) of Co4 cluster. System

Eb (eV)

εd (eV)

Δq (e)

Mag (μB)

dCo–C (Å)

dCo–Co (Å)

Co4/PG Co4/SVG Co4/BVG Co4/NVG Co4/PVG Co4/B-SV Co4/3B-SV Co4/N-SV Co4/pyridinic (3 NSV)

1.30 7.24 3.01 0.85 3.42 6.16 4.65 5.62 4.17

−2.29 −2.57 −2.23 −2.07 −2.34 −2.50 −2.44 −2.47 −2.91

0.93 1.09 1.07 0.98 1.08 0.73 0.41 1.00 1.18

6.2 6.0 7.0 7.0 6.9 5.1 7.0 5.0 7.3

2.03 1.81 2.18 2.18 2.14 1.82 1.83 1.81 1.83

2.33 2.32 2.28 2.34 2.30 2.31 2.31 2.35 2.29

3.2.3. Doping heteroatom in vacancy of SVG We further considered the doping of heteroatom B, N and P in the monovacancy site of graphene, denoted as BVG, NVG and PVG, according to the positive effects in tuning hydrogen storage performance as discussed earlier [23,53]. It was found that the support with single dopant B or P atom largely stabilizes the adsorption of Co4 with binding energy of 3.01 and 3.42 eV, respectively, in contrast to a prominent weakening effect of N-doped graphene (0.85 eV) relative to PG (1.30 eV). Analogously, there would be five, five and four H2 molecules adsorbing on Co4/BVG, Co4/NVG and Co4/PVG, respectively, after the first two hydrogen molecules dissociatively adsorbing over the three complexes. Moreover, trivial changes are found for the adsorbed configurations and structural parameters over heteroatom-doped supports in comparison with that on PG (see Fig. 5 and Table S4). It is noticed in Fig. 5a that doping of B atom in graphene shows better performance than that of N- or P-doped in modulating hydrogen adsorption strength with three H2 adsorption in the reversible energy region. Generally, Bdoped graphene stands out as the support of Co4 for hydrogen storage, by virtue of its good stability, favorable hydrogen adsorption strength and enhanced storage capability.

the molecular form, and the adsorption energy gradually declines as seen from Fig. 3a. It is saturated with an adsorption energy of 0.57 eV/ H2 until another five H2 molecules are adsorbed on Co4/PG precovered with two dissociative H2, i.e. the configuration of Co4/PG@(H2)5(H)4 in Fig. 3d. Similar case was reported over Pd4/PG with the maximum of five hydrogen molecules with Eads = 0.31 eV/H2 [29]. As shown in Fig. 3 and Table S2, metallic Co4 cluster maintains its tetrahedron structure over PG upon successive hydrogen adsorbing, and the CoeCo bond are gradually elongated in the range of 2.33–2.47 Å relative to that on free Co4, as well the nearest CoeH and HeH bonds are barely affected with values limited in 1.66–1.69 and 0.84–0.88 Å, respectively. The gain of electrons of adsorbates and the total magnetic moment are reduced gradually with the increase of hydrogen number. 3.2.2. Introducing monovacancy in PG It was reported that vacancy in PG can modify the interaction between Co4 and grapheme [41]. Herein, we take Co4 adsorbed on monovacancy graphene as a representative and the calculations in Fig. 4 present the similar adsorption energy varying with the number of 5

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Fig. 6. (a) The average adsorption energy (Eads) of H2 varying with its number on Co4 supported by B-SV, N-SV, 3B-SV and 3N-SV (pyridinic) sheets; (b)–(d) top and side views for the configurations of Co4/ pyridinic@(H2)M(H)4 (M = 0, 4, 6). (Color map: blue–cobalt, pink–hydrogen, gold–carbon, green–boron). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

according to the literatures [23,25,53,54]. Firstly, we found from Table 1 that the adsorbing of Co4 on these supports was greatly strengthened but inferior to that on SVG (7.24 eV), with binding energy around 6.16, 4.65, 5.62 and 4.17 eV for B-SV, NSV, 3B-SV and pyridinic, respectively. Upon hydrogen adsorbing, the results in Fig. 6 indicate that it attains the saturated configurations with the first two dissociative and additional six H2 molecules adsorbed over these composites. By comparison, we notice that more dopants in graphene actually give rise to much weaker adsorption of hydrogen with the hydrogen number less than six. Moreover, one can see from Fig. 6a and Table S5 that combination of monovacancy with heteroatom B/Ndoping in graphene gives rise to the maximum hydrogen uptake of eight H2 over supported-Co4. Especially, there would be three H2 adsorption fall in the reversible energy window over the Co4/N-SV and Co4/pyridinic complexes, same as to that of Co4/BSV. As discussed previously [6], we further checked the case that cluster Co4 anchored on the graphene sheet with three B and seven N atoms co-doped adjacent to a divacancy (see Fig. S3), and it is found to be undesirable for hydrogen storage because of the unfavorable adsorption energy of 0.63 eV/H2 at the saturation. Fig. 7. Calculated binding energies (Eb) of Co4 adsorbed on various graphene supports and the corresponding d-band center (εd) values of Co4, together with the maximum hydrogen uptake number and the number of reversible H2 molecules.

3.3. Analysis of hydrogen storage ability and comparison with other analogues

3.2.4. Embedding heteroatom and vacancy in PG In view of the positive effects of monovacancy and B/N-doping in graphene as aforementioned, an interesting question arises naturally: how would the supported-Co4 behave over the graphene sheet co-existed with heteroatom and vacancy? Thereby, a great deal of calculations were performed for multiple hydrogen adsorption over Co4 supported on such defective graphene sheets as substituting atomic C by one, three B or N atoms in proximity to carbon monovacancy, abbreviated as B-SV, N-SV, 3B-SV and pyridinic (3N-SV), respectively,

To clarify the hydrogen storage capability of supported-Co4, we displayed the related numerical results for electronic d-band center (εd), binding-energy of Co4, and hydrogen uptake number of all the Co4/GRA composites in Fig. 7. Apparently, the point defects in graphene contribute to enhancing the stabilities of Co4/GRA composites, with an exception of N-doped support (NVG). The d-band center (εd) of supported-Co4 obeys an increasing trend with the support: pyridinic < SVG < B-SV < N-SV < 3B-SV < PVG < PG < BVG < NVG, with the εd values increasing from −2.91 to −2.07 eV. Approximately, the enhanced stabilities of Co4/GRA composites are relating to the relatively lower d-band center of Co4, larger hydrogen uptake in reversible 6

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Fig. 8. The projected density of states (DOS) of H(H2)-1s (red line), Co4-3d (blue line) and C(N)-2p (black line) for the configurations: (a) Co4/pyridinic@(H)2 and Co4/pyridinic@(H2)M(H)4 (M = 1, 3, 6), (b) Co4/N-SV@(H)2 and Co4/N-SV@(H2)M(H)4 (M = 1, 3, 6). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

relative to that of hydrogen absence, respectively (See Table S5). Furthermore, Fig. 9 displays the computated binding energies of metal clusters supported on different substrates, and their maximum hydrogen uptake [29,30,47,52,54–57] for a comparison. As one can see from the left part of Fig. 9 that the hydrogen storage capability of Ti4 and Pd4 supported on penta-graphene [30] outperform the other cases over pristine supports [29,52,55,56] including Co4/PG, because of the better cluster stabilities and the larger hydrogen uptake around 7 H2. It is worth mentioning that the case of Ti4 on pristine graphene [58,59] is able to absorb up to seven hydrogen molecules including two H2 adsorbed dissociatively, along with Ti4 adopting a planar configuration. We also note that over monovacancy graphene hydrogen storage

hydrogen adsorption energy. Moreover, Fig. 8 presents the spin-polarized density of states (DOS) for hydrogen adsorbed over Co4 anchored on pyridinic and N-SV supports, and one can observe that apparent hybridization between Co4-3d and H(H2)-1s orbitals is dominantly located in the energy range from −6.0 to −4.0 eV, including the Kubas-interaction [47] between the σ and σ* orbitals of H2 interacting with metal orbitals, and becomes less prominent with hydrogen molecules aggregating. Meanwhile, the DOS plots of metallic Co4, donating electrons to hydrogen molecules, illustrate a noticeable reduction in total magnetic moments of metal cluster with multiple hydrogen adsorption, and it declines by ~86.3% and 83.2% on pyridinic and N-SV sheet at the saturated configurations 7

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Fig. 9. Comparison of the maximum number of adsorbed hydrogen over supported metal clusters, along with the metal cluster binding energy (Eb). Other theoretical data are cited from: Pd4/SVG [47], Pd4/PG [29], Pd4/DVG [29], Ni4/Penta-GRA [30], Pt4/Penta-GRA [30], Ti4/Penta-GRA [30], Pd4/Penta-GRA [30], Pt4/PG [53], Pt4/BVG [53], Pt4/BC3-GRA [53], Ni4/SVG [48], Ti4/SVG [48], Pt4/CNT [55] (Carbon Nanotube), Rh4/Zoelite [56], Pt4/Zoelite [56], Pt4/SV-NH [57] (SV-NH: single-vacancy-carbon nanohorn).

Acknowledgements

capability of Co4 prevails over the other analogous i.e. Pd4 [47], Pt4, Ti4, and Ni4 [54], by virtue of the largely enhanced stability and the adsorbed hydrogen number amounting to 8H2. Heteroatom B embedding in the monovacancy of graphene (BVG) yields similar hydrogen uptake of the supported-Pt4 [52] and Co4, but with small discrepancy in their stabilities bound to BVG support. Promisingly, Co4 decorated on pyridinic realized experimentally or the defective graphene support with substitutional N atom adjacent to C-vacancy (N-SV) can act as a desirable hydrogen storage medium, on account of their better stabilities and the maximum hydrogen uptake up to 8 H2, including reversibly adsorbed three H2. As reported experimentally [34], present calculations verify that graphene sheet can efficiently modulate the interaction between Co4 and hydrogen by defects in support, facilitate nanoparticle cobalt serving as hydrogen storage material, especially the defective graphene with atomic N and C-vacancy co-existing.

This work was supported by the Natural Science Foundation of Henan Province (No. 182300410203) and the Basic Research Program of Education Bureau of Henan Province (No. 18A140004 and 19A140004). We also thank the High Performance Computing Center of Henan Normal University for providing computing resources. Appendix A. Supplementary material A variety of defective graphene supports, energy profiles for H2 molecule dissociation on free-standing Co4, average adsorption energy of hydrogen over Co4/3B7N-DVG (DVG: 555-777), comparison of the hydrogen adsorption energies calculated with and without vdW correction, numerical results for hydrogen adsorbing over different compounds: Co4/PG, Co4/SVG, Co4/BVG, Co4/B-SV and Co4/3B-SV, Co4/NSV and Co4/pyridinic (3N-SV). Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2019.144413.

4. Conclusions In summary, we have studied hydrogen adsorption on Co4 supported over a series of graphene sheets by means of first-principles calculation. It is found that the defects in graphene sheets can efficiently modulate the hydrogen storage capacities by modifying the interaction between Co4 and the support. For instance, the pristine support largely weakens the adsorption strength of hydrogen with Co4 relative to the free case, and introducing carbon-vacancy in graphene greatly stabilizes the metal cluster and enhances hydrogen uptake. Specifically, Co4 decorated on pyridinic sheet (3N-SV) and N-SV, stand out as suitable hydrogen storage materials, attributing to their excellent stabilities and favorably reversible hydrogen (3 H2) adsorption. Meanwhile, it is theoretically predicted that the better hydrogen storage performance is approximately relating to the lower d-band center of Co4 and the moderate interaction strength between Co4 and defective graphene. This work explores the possibility of Co4-decorated defective graphene sheets as hydrogen storage materials, enriching the metaldecorated 2D nanomaterial hosts for hydrogen further.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 8

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