S. R. Naqvia,
T. Hussain*c,
P. Panigrahid,
W. Luoa and
R. Ahujaab
aCondensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden
bApplied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden
cCentre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, Australia. E-mail: t.hussain@uq.edu.au
dCentre for Clean Energy and Nano Convergence (CENCON), Hindustan University, Padur, Kelambakkam, Chennai, India
First published on 26th January 2017
We report, for the first time we believe, a detailed investigation on hydrogen storage efficiency of scandium (Sc) decorated boron carbide (BC3) sheets using spin-polarized density functional theory (DFT). We analyzed the energetics of Sc adsorption and explored the most favorable adsorption sites of Sc on BC3 sheets with 3.12%, 6.25%, and 12.5% coverage effects. Our investigations revealed that Sc strongly binds on pristine BC3 sheet, with a minimum binding energy of ∼5 eV, which is robust enough to hinder Sc–Sc metal clustering. Sc, the lightest transition metal, adsorbs a large number of H2 molecules per atom, resulting in a reasonable storage capacity. With 12.5% Sc-coverage, functionalized BC3 sheets could attain a H2 storage capacity of 5.5 wt% with binding energies suitable for a practical H2 storage medium.
Carbon-based nanomaterials, owing to their low cost, light weight, large surface area and novel H2 adsorption, have emerged as a subject of further research interest.7–15 However, in their pristine forms such nanomaterials show very feeble binding affinities as a result of weak van der Waals interactions between H2 and the host material.16,17 For practical purposes the binding energy of H2 with the respective storage material should be in the range of −0.2 eV to −0.6 eV.18 To achieve this criterion, metal decorated carbon-based nanomaterials have been investigated as potential candidates for H2 storage. The binding affinity could be further tuned by introducing isolated charged sites. However, the bonding mechanism and the binding enthalpy for the interaction of H2 with metal adatom depend on the type of metallic site, the extent of polarizability and on charge localization. It has been shown theoretically that charge transfer between the alkali metal (AM) centers and the host material leaves the AM adatom in the cationic state. The H2 adsorption enthalpy is enhanced due to transfer of charge between H2 and the metallic species.19 A rapid decrease in adsorption energy due to increasing temperature has also been reported, which limits the room temperature H2 storage of AM functionalized carbonaceous materials.20–23 To enhance the binding aptitude of H2, both alkaline earth metal (AEM) and transition metal (TM) coated materials have been extensively studied in the recent past.26–34 Smaller adsorption energies of H2 for AEM than TM decorated storage media have been reported.26 Functionalization with TMs leads to strong hybridization between the hydrogen σ- or σ*-orbitals and low-lying empty d-orbitals in the TM atoms.19 This kind of interaction is typically observed in the Kubas complexes,24,25 and is responsible for enhanced binding affinity towards H2 molecules. However, heavier TMs lead to lower gravimetric density for H2 storage. In addition, clustering is also a crucial problem for TM-doped carbon-based materials, leading to lower storage efficiency.27,29 Thus, H2 storage efficiency could be drastically improved by using light TM atoms and further preventing the metal clustering effect.
In this context, Nachimuthu et al. have reported a H2 storage capacity of 6.4 wt% in Ni–Ti–Mg functionalized boron-doped graphene. Their results indicate that the boron content in graphitic materials could successfully hinder the formation of metallic clusters.26,33 In contrast, Beheshti et al. suggested that substitutional doping of boron atoms in graphene also efficiently prevents Ca atoms from forming clusters, which is unavoidable in the case of pure graphitic materials.30
Recent investigations on boron carbide (BC3) nanomaterials in pure,35 titanium-doped,32 calcium-doped42 and lithium-doped34 systems have urged the scientific community to unravel the potential of this novel material for the purpose of practical H2 storage. In the present work, we are interested in exploring the H2 storage potential of scandium (Sc) functionalized boron carbide (BC3) nanosheets using spin-polarized density functional theory (DFT) calculations. Our investigations include structure analysis, bonding characteristics, charge transfer mechanisms and hydrogenation of functionalized BC3 sheets. The novelty of the present study lies in the fact that even in its pristine form BC3 sheet holds multiple Sc dopants with much higher binding energies than graphene, graphane, h-BN sheet, silicene and many other materials, which make this material superior to these 2D materials.
To functionalize pure BC3 sheet with Sc atoms, we have investigated the adsorption behavior of a single Sc atom on all available high symmetry sites on the single side of the sheet. The six available adsorption sites for the Sc atom are designated (i) top of B atom (T1), (ii) top of C atom (T2), (iii) hollow site in BC ring (H1), (iv) hollow site in C ring (H2), (v) bridge between B and C atoms (B1) and (vi) bridge between two C atoms (B2), as shown in Fig. 1. For each possible site, the Sc atom is initially kept at a vertical distance of 2 Å. However, the Sc atom initially on T2 and B2 sites migrates towards the more favorable H2 site. The Sc atom binds to H2 sites with a binding energy of 7.11 eV at a vertical distance (dSc–sheet) of 1.45 Å. Initial and final geometries of all the possible configurations of Sc on BC3 are given in S1 (ESI†). The adsorption energies for all Sc occupied sites are presented in Fig. 1(b). The adsorption energy of the Sc atom is defined as:
Eb = [E(BC3) + E(Sc)] − E(BC3@Sc) | (1) |
The strong binding of single Sc on BC3 encouraged us to introduce more Sc dopant on the sheet. Thus, in a similar fashion, we found the most favorable site for the second Sc atom (6.25% Sc-coverage) on the BC3 sheet, as shown in Fig. 2. The most preferred binding site for the second Sc atom turns out to be the hollow site of the C ring (H2) exactly opposite to the first Sc atom. The binding energy per Sc is found to be 6.02 eV, which is slightly lower than for 3.12% Sc-coverage. However, it is still much higher than Sc–Sc cohesive energy, indicating that BC3 could be further exposed to Sc adatoms. The distance dSc–sheet has now increased by 0.19 Å due to the weakening of the bond as compared to the single Sc case (see Table 1).
System | Binding energy per Sc Eb (eV) | Average Sc to BC3 distance dSc–sheet (Å) | Minimum dopant–dopant distance dSc–Sc (Å) | Charge per Sc (e−) |
---|---|---|---|---|
BC3–1Sc | 7.11 | 1.45 | — | +1.444 |
BC3–2Sc | 6.02 | 1.64 | 3.28 | +1.390 |
BC3–4Sc | 5.45 | 1.95 | 3.65 | +1.251 |
In the case of 4 Sc on BC3 sheet (12.5% Sc-coverage), the most favorable binding configuration is shown in Fig. 2(c). The optimized geometry of this configuration is found to be a little degraded owing to the relatively high Sc doping. Here the binding energy per Sc is found to be 5.45 eV, still almost 40% higher than Sc–Sc cohesive energy. The energetics analysis reveals that even at a significantly high Sc doping concentration, the metal dopants bind strongly enough to nullify cluster formation and make a uniform distribution over the BC3 monolayer.
The bonding mechanism between Sc and BC3 monolayer could be explained by studying the charge transfer mechanism. We employed Bader charge analysis for this purpose, which depicts a transfer of 1.251e− of charge from Sc atom to BC3 sheet due to the lower electronegativity of the former than the latter. The bulk portion of this transferred charge goes to the C atoms of BC3 that are in close vicinity to Sc dopant. This would transform Sc into a partially positive charged state, which proves to be extremely helpful in anchoring the incident H2 molecules. The accumulation and depletion of charges were described by plotting the isosurface charge densities for 3.12%, 6.25% and 12.5% Sc-coverage, as shown in Fig. 3(a–c). The charge density can be calculated by the following relation:
Δρ = ρ(BC3@nSc) − ρ(BC3) − ρ(nSc) | (2) |
{n = 1, 2, 4} |
The minimum distance between Sc atoms (dSc–Sc) in the case of 2 Sc (6.25% coverage) and 4 Sc (12.5% coverage) doping is found to be 3.28 Å and 3.65 Å respectively. The binding energy per Sc atom decreases, whereas the optimum distance between the Sc and BC3 sheet increases with the increase in Sc dopant. Though the bond elongation for increased Sc-coverage (dSc–sheet) justifies comparatively weaker binding between the sheet and Sc atoms, the binding is still strong enough to prevent the formation of Sc–Sc clusters. Results from structural and charge analysis are presented in Table 1.
Eads = [E(BC3@mSc + nH2) − E(BC3@mSc) − nE(H2)]/n | (3) |
The H2 gravimetric density (storage capacity) can be calculated by the following relation:
H2 storage capacity = NH × WH/[(NB × WB) + (NC × WC) + (NSc × WSc) + (NH × WH)] |
In the above relation N represents the number of each type of atoms and W represents the molecular weight of each element present in the system.
The number of H2 molecules around Sc was gradually increased from 1 to the maximum adsorption limit. For the lowest Sc-coverage (3.12%), a maximum of five H2 molecules per Sc atom could be adsorbed to the functionalized sheet, which resulted in a relatively small H2 storage capacity of 2.364 wt%. Top and side views of the lowest energy hydrogenated systems are represented in Fig. 5(a) whereas all the other possible configurations are given in S2 (ESI†). The average distance between adsorbed H2 and Sc atoms (dH2–Sc) and the average binding energy for maximum H2 uptake are 2.2 Å and −0.34 eV respectively.
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Fig. 5 Hydrogenated BC3 sheets. Top view and side view for (a) 1 Sc [3.2% Sc] and (b) 2 Sc [6.4% Sc]. (c) Extended top and side view for 4 Sc [12.5% Sc]. |
It is important to mention here that the H2 adsorption energies are quite sensitive to van der Waals functionals, so a careful choice of a functional needs to be made in order to achieve unvarying values of adsorption energies. The DFT-D2 method of Grimme used in the present study has been extensively used and it has yielded reliable results consistent with the literature.45–48 For a comparison we have calculated the adsorption energies of H2 and 2 H2 adsorbed on BC3@Sc by using the non-local van der Waals functional optPBe-vdW49 and found that the adsorption energies of H2 and 2 H2 around BC3@Sc are −0.310 eV and −0.293 eV respectively. These values are roughly 23% lower than those calculated by DFT-D2.
The bond length of H2 has increased slightly to 0.76 Å. In the case of two Sc atoms, each Sc could bind 5 H2 (shown in Fig. 5(b)), which yielded a H2 storage capacity of 4.184 wt%, significantly higher than that of the BC3@Sc system. The isosurface charge density of BC3@Sc–H2 and BC3@Sc–5H2 is shown in S3 (ESI†). The adsorption energies of H2 with the functionalized BC3 material again show a decreasing trend with an increasing number of H2. However, the binding energy of H2 for maximum hydrogen coverage is −0.3 eV, which still lies in the energy window above weak physisorption (Fig. 6). The average distance between each H2 and Sc atom is 2.39 Å and the average bond length of H2 is 0.77 Å. We employed a similar procedure to hydrogenate the BC3 sheet with 12.5% Sc-coverage. In total, 16 H2 molecules could be incorporated in the functionalized system, giving a gravimetric density of 5.5 wt% (Fig. 5(c)).
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Fig. 6 Adsorption energies of H2 on functionalized BC3 sheets for three different values of Sc-coverage. |
Interestingly, the first H2, introduced in between 2 Sc atoms (on each side of the sheet), experiences dissociative adsorption. The two H atoms locate to an optimal distance apart (dH–H) of 2.2 Å and each H attains an electronic charge of −0.495e−. The bond dissociation leads to adsorption of one H per Sc atom (H/Sc) at an average distance (dH–Sc) of 1.93 Å and average binding energy of −0.9 eV. Further addition of H2 in the vicinity of Sc atoms leads to associative adsorption of H2. In the first step, the H2 introduced in between 2 Sc atoms gets dissociated due to the presence of enough charge to contribute to the σ*-antibonding orbitals. To support this argument, Bader charge analysis also reveals partially polarized H2 molecules in the vicinity of Sc atoms. However, for successive adsorption of H2, there is not enough charge available to destabilize the H2 molecule. It is evident from Fig. 6 that for each Sc-coverage value, the average adsorption energy of H2 monotonically decreases with increasing number of H2 molecules.
By gradually increasing the number of H2 from 1 H2/Sc to the maximum limit of adsorption, the average binding energy for associative adsorption varies from −0.31 to −0.20 eV for 12.5% Sc-coverage (Fig. 6). The average bond length of adsorbed H2 molecules increased to 0.78 Å which is slightly larger than for free H2 (0.75 Å). Bond elongation is typically observed in Kubas interaction where an amount of charge is donated from H2 to the empty d-orbital of TM followed by back-donation of charge from TM to an anti-bonding orbital of H2. Such donation and back-donation of charge governed by Kubas interaction has been extensively studied in the literature.8,50–52 The greater charge transfer results in a longer bond length of H2. Successive addition of H2 molecules to the functionalized BC3 sheet (with 12.5% Sc-coverage) reveals that a total of 15 H2 (including 2 dissociated molecules) could be adsorbed in the vicinity of Sc atoms, where one H2 directly binds to the sheet at a distance (dH2–sheet) of 2.7 Å above the B2 site. Top and side views of the functionalized system with maximum H2 uptake and average adsorption energies with increasing number of H2 are represented in Fig. 5(c).
The functionalized systems (BC3@Sc, BC3@2Sc, BC3@4Sc) reported in the present work could achieve a maximum H2 storage capacity of 5.5 wt%, which fulfills the DOE target to be met by the year 2017. However, the H2 capacity is smaller than given in a paper by Durgun et al.,7 which reported on the H2 storage properties of selected transition metals (Ti, Sc, V) decorated on carbon chain, graphene and carbon nanotubes, which attained high H2 storage capacities. However, to ensure the uniform distribution of metal dopants over the host materials, the binding energies of the former must be higher than their respective cohesive energies. In the paper mentioned above, the binding energies of TM dopants (Ti, Sc, V) fall short of their respective cohesive energies, which could allow the formation of TM clusters which significantly reduces the H2 storage capacity.
The novelty of our work lies in the fact that, despite lower H2 storage capacity as compared with the Durgun et al. paper, Sc, 2 Sc and 4 Sc bind strongly with BC3, nullifying the possibility of cluster formation and ensuring the formation of stable doped systems.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24890j |
This journal is © The Royal Society of Chemistry 2017 |