First-principles investigation of the effects of Ni and Y co-doped on destabilized MgH₂

Abstract

The Ni and Y co-doping effect on the structural stabilities and dehydrogenation properties of destabilized MgH₂ was studied by first-principles calculations. Ni and Y dopants prefer to occupy the Mg3 and Mg2 positions due to the minimal total electronic energy. The formation enthalpy was used to evaluate the stability of the doped MgH₂ systems. Most Ni and Y co-doped MgH₂ systems are more stable than Ni single-doping. In particular, the case of x = 20% (Mg₈Ni₈Y₂H₃₆) exhibits the highest stability. During the dehydrogenation process, the Ni and Y co-doped MgH₂ system possesses promising dehydrogenation properties compared with pure Ni doping, which can be attributed to the relatively lower hydrogen desorption enthalpies. The electronic structures show that hybridization of the dopants with Mg and H atoms can strongly weaken the Mg–H interactions, which effectively improve the dehydrogenation properties of the Ni and Y co-doped MgH₂ system.

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1. Introduction

Metal hydrides with a high hydrogen storage capacity and demonstrated cycling capability,^1–4 are considered to be some of the most promising hydrogen storage media for automotive applications. Among these hydrogen storage materials, magnesium hydride (MgH₂) has been investigated extensively in the last two decades due to its high hydrogen storage capacity of 7.6 wt% and good reversibility,^5–7 together with the cheap cost and light weight of magnesium.^8,9 Unfortunately two technical obstacles limit the practical application of MgH₂: (i) the high thermodynamic stability, which is responsible for the high dehydrogenation temperature requirement of 573 K at 1 bar H₂.^10,11 (ii) The poor kinetics in the hydrogenation and dehydrogenation reaction, which can be attributed to the low dissociation rate of H₂ on the metallic Mg surface, strong Mg–H bonding in magnesium hydride, and slow hydrogen diffusion ability in MgH₂.^10–12

To overcome these limitations in MgH₂, various efforts have been made in the design of Mg-based hydrogen storage materials. On the one hand, reducing the gain size is an effective way to improve the thermodynamic behavior of MgH₂ by shortening the diffusion length, introducing defects and increasing the surface areas.^13,14 Theoretical calculations predicted that the reaction enthalpy of MgH₂ nanoparticles is reduced compared with the bulk material only when the particles are smaller than 2 nm and contain <50 Mg atoms.^15,16 However, this property is not straightforward for practical applications because it is extremely difficult to prepare nanoparticles of this size and keep the stabilities of these nanoparticles after repeated hydrogenation/dehydrogenation cycles.¹⁷ On the other hand, introducing catalysts such as transition metals and transition metal oxides will improve the hydrogen adsorption/desorption kinetics and thermodynamic properties of MgH₂. It has been revealed that doping MgH₂ with transition metals can improve its hydrogen adsorption/desorption kinetics as well as its thermodynamic properties. Many transition metals (TM) such as Ni, Ti, V, Zr, Fe, Ru, Co, Y, Rh, Pd, Cu, Ag and Nb, have been used as additives for MgH₂.^18–24 Ren et al. found that the V dopant can significantly reduce the dehydrogenation temperature and improve the kinetics of MgH₂.²⁰ Computational studies by Takahashi et al. used density functional theory (DFT) to investigate the hydrogenation and dehydrogenation mechanism of Nb, NbO and Nb₂O₅ doped on MgH₂, and found that these dopants have a significant catalytic effect on the dehydrogenation of MgH₂.^21,22 Dai et al. carried out first principles calculations based on DFT to show that Ni can improve the dehydrogenation properties of MgH₂ by the weakened Mg–H bond.¹⁰ Very recently, Hudson et al. discovered that graphene decorated Fe clusters dramatically improved the kinetics of MgH₂, and they showed that non-transition metals like graphene can be used as a catalyst for the MgH₂ system.²⁴

Previous studies^25–28 demonstrated that multiple transition metal additions generally have a better effect in improving the hydrogenation performance of MgH₂ than addition of a single transition metal. It was found that co-doped MgH₂ showed excellent hydrogen absorption/desorption kinetics, especially in the latter process, since it reduced the activation energies of both processes and weakened the Mg–H bonds. Zaluska et al. reported that the best storage kinetic results are contributed by a combination of the transition metals, such as V + Zr or Mn + Zr.²⁵ Another promising material is the Mg–Ni–Y alloy, which can reach an invertible hydrogen storage capacity of up to 5.3 wt% H₂. The rates of hydrogenation and dehydrogenation are up to 1 wt% H per min at a temperature of 250 °C, with a remaining nanocrystalline structure even after several cycles of H₂ uptake and release.²⁶ Li et al. investigated Mg alloyed 20 wt% Ni–Y and observed high gravimetric hydrogen storage densities and excellent hydrogen sorption kinetics. At 293 K and 473 K under 3.0 MPa H₂, it can absorb 4.16 and 5.59 wt% hydrogen, respectively. It can also desorb 4.75 wt% hydrogen in 15 min at 573 K under 0.1 MPa H₂.²⁷ Zhou et al. observed that Al and Y co-doped MgH₂ can weaken Mg–H bonds and promote hydrogen dissociation and desorption.²⁸ Moreover, the exact mechanism of Ni combined with Y as a catalyst in the enhancement of hydrogen storage properties of MgH₂ is not completely understood. Therefore, it is necessary to develop a systematic investigation of the synergistic effects of Ni with Y on improving the hydrogen storage properties of MgH₂.

In this paper we performed a systematic study on the collaborative effects of Ni and Y co-doped with destabilized MgH₂ using first-principles calculations. The preferential sites of Ni and Y dopants on MgH₂ were determined by the lowest total electronic energy. The formation enthalpies and hydrogen desorption enthalpies were used to study the dopants’ influence on the structural stability and dehydrogenation properties of MgH₂. Electronic structures were analyzed to identify the intrinsic mechanisms of the dopants’ influence on the bonding and dehydrogenation properties of the destabilized MgH₂ matrix.

2. Computational methods

Energy and electronic structure calculations were performed under the framework of density functional theory (DFT) via the Vienna Ab initio Simulation Package (VASP) code.^29,30 The projector augmented wave (PAW) method was used to spin out the valence electron density; the generalized gradient approximation (GGA) in the scheme of Perdew–Wang 91 (PW91) was adopted for the exchange–correlation functional.^31,32 For the plane wave basis set a cutoff energy of 350 eV was used throughout. The model of the co-doped MgH₂ with Ni and Y was simulated by 3 × 3 × 1 and 3 × 3 × 3 supercells. The Brillouin-zone samples were 3 × 3 × 7 and 3 × 3 × 3 Monkhorst–Pack k-point meshes for the supercells above, respectively. The electronic structures were defined as self-consistent if the differences between two consecutive energies and forces were less than 10⁻⁷ eV and 0.01 eV Å⁻¹.

3. Results and discussion

3.1. Calculation models and site preference

MgH₂ has a tetragonal structure (P4₂/mnm, group no. 136) with experimentally measured lattice parameters of a = 4.501 Å and c = 3.010 Å.³⁰ Two Mg atoms occupy the 2a (0, 0, 0) site and four H atoms occupy the 4f (0.303, 0.303, 0) site. In a previous work, we calculated that the lattice parameters for a unit cell of MgH₂ are a = 4.477 Å and c = 2.989 Å, which are very close to the experimental³⁴ and other theoretical results.^33,35,36 The model of MgH₂ was built from a 3 × 3 × 1 supercell and computed using those bulk parameters. The 3 × 3 × 1 supercell (see Fig. 1) contained a total of 54 atoms with four non-equivalent positions for Mg and six non-equivalent positions for H. The optimal atomic positions of Mg and H atoms are in good agreement with the theoretical data.²⁸

Fig. 1 Top (a) and side (b) views of the MgH₂ 3 × 3 × 1 supercell model. Mg1, Mg2, Mg3 and Mg4 denote the four non-equivalent positions for Mg, respectively.

In order to find the optimum geometry and doping sites of dopants (Ni and Y) in MgH₂, each of the four non-equivalent positions of Mg were substituted by Ni in order. The calculated total electronic energies of the Ni doped system are shown in Fig. 2(a). These results suggest that the Ni atom prefers to stay in the Mg3 position, due to the minimal total electronic energy. The new compounds are denoted as (Mg, Ni)H₂. Then, Mg is substituted by Y in the other three non-equivalent positions (Mg1, Mg2 and Mg4) of the (Mg, Ni)H₂ compound. The calculated total electronic energies are shown in Fig. 2(b). The Y atom is likely to substitute in the Mg2 position due to its lowest total electronic energy. The new compounds are denoted as (Mg, Ni, Y)H₂. Adopting the same method, we also calculate the total electronic energies of different doping sites of dopants (Ni and Y) in the MgH₂ 3 × 3 × 3 supercell. The calculated total electronic energies of these doping systems are shown in Fig. 2(c) and (d). Ni and Y prefer to occupy the Mg3 and Mg2 positions in the 3 × 3 × 3 supercell, respectively. The calculated results of the 3 × 3 × 3 supercell are consistent with the results of the 3 × 3 × 1 supercell; we have chosen the latter results in the following calculations due to the 3 × 3 × 1 supercell model having a higher calculation efficiency.

Fig. 2 The total electronic energy for: (a) doping MgH₂ (3 × 3 × 1 supercell) with Ni in four non-equivalent positions of Mg; (b) doping (Mg, Ni)H₂ (3 × 3 × 1 supercell) with Y in the other non-equivalent positions of Mg; (c) doping MgH₂ (3 × 3 × 3 supercell) with Ni in four non-equivalent positions of Mg; (d) doping (Mg, Ni)H₂ (3 × 3 × 3 supercell) with Y in the other non-equivalent positions of Mg.

In order to systematically investigate the role of Ni and Y co-doping on the structural stability of MgH₂, we studied the substitution of Mg2 by Y in different doping concentrations. We chose eight doping concentrations of x = 0, 10%, 13%, 20%, 27%, 30%, 33% or 40% in (Mg, Ni)H₂, which meant substituting n = 0, 1, 1.3, 2, 2.7, 3, 3.3 or 4 out of 10 Mg atoms in Mg_10−nNi₈Y_nH₃₆. For the case of 1.3, 2.7 or 3.3 out of 10 Mg atoms in Mg_10−nNi₈Y_nH₃₆, we performed an equivalent treatment by replacing 4, 8 or 10 of 30 Mg atoms in the (Mg, Ni)H₂ (3 × 3 × 3) supercell to achieve the doping level of x = 13%, 27% or 33% in (Mg, Ni)H₂. The chosen method of preferential positions in different concentrations is consistent with that used in the former.

The favorability of single-doping and co-doping in MgH₂ can be identified by the substitution energies (E_sub), which were calculated via the following definition:^37–39

E_sub = 1/54[E_t(Mg_10−nNi₈M_nH₃₆) − E_t(Mg₁₈H₃₆) − 8E_b(Ni) − nE_b(Y) + (8 + n)E_b(Mg)] (1)

where E_t(M) refers to the total energies of hydrides in the supercells. E_b represents the total energies per atom in the bulk structure. The obtained substitution energies of the doped materials with different Y concentrations are presented in Fig. 3. It can be seen that Ni and Y co-doped MgH₂ systems have a lower substitution energy value compared with the Ni single doped system, except for the doping concentrations of x = 10% (Mg₉Ni₈YH₃₆) and 33% (Mg_6.8Ni₈Y_3.3H₃₆). Compared with the Ni single-doped system, the substitution energy of Mg₉Ni₈YH₃₆ and Mg_6.8Ni₈Y_3.3H₃₆ are slightly increased by about 2.6% and 2.9%, respectively. Furthermore, the lowest substitution energy value is located at a doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆), which is lower than that of the Ni single-doping by 65.9%. From an energy point of view, the smaller substitution energy corresponds to a more favorable substitution doping. Hence, most of the co-doping of Ni and Y into MgH₂ is more energetically favorable than the single-doping. In particular, x = 20% (Mg₈Ni₈Y₂H₃₆) is the most energetically favorable doping.

Fig. 3 The calculated substitutional energies (E_sub) of the doped hydrides with different Y concentrations of x = 0, 10%, 13%, 20%, 27%, 30%, 33% or 40%.

3.2. Stability and dehydrogenation properties

Commonly, the structural stability of a crystal can be evaluated by its formation enthalpy ΔH_form. A negative formation enthalpy indicates that the crystal can exist stably. Besides, a lower formation enthalpy suggests a stronger stability.⁴⁰ In order to investigate the stability of Ni and Y doping on the MgH₂ system, the formation enthalpies ΔH_form are calculated by using eqn (2):^28,40,41

ΔH_form = 1/54[E_t(Mg_10−nNi₈Y_nH₃₆) − 8E_s(Ni) − nE_b(Y) − (10 − n)E_b(Mg) − 18E(H₂)] (2)

where E_t(M) refers to the total energies of hydrides in supercells. E_b represents the total energies per atom in the bulk structure. The total energy of the free H₂ molecule, E(H₂), was computed as −6.77 eV using a 8 ų cubic cell, consistent with other theoretical results.^28,47 The calculated formation enthalpy of the doped hydrides with different Y concentrations are presented in Fig. 4. The formation enthalpies of these eight compounds are all found to be negative, which indicates they can exist stably. Ni and Y co-doped MgH₂ systems have a lower formation enthalpy value compared with the Ni single-doped system, except for the doping concentrations of x = 10% (Mg₉Ni₈YH₃₆) and 33% (Mg_6.8Ni₈Y_3.3H₃₆). Compared with the Ni single-doped system, the formation enthalpy of Mg₉Ni₈YH₃₆ and Mg_6.8Ni₈Y_3.3H₃₆ are increased by about 6.5% and 17.8%, respectively. Furthermore, x = 20% (Mg₈Ni₈Y₂H₃₆) has the lowest enthalpy value, which is lower than that of Ni single-doping by 172.6%. This trend is consistent with the calculated results of the substitution energy. Hence, the doping of Ni combined with Y into MgH₂ exhibits a higher stability than Ni single-doping. In particular, the doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆) exhibits the highest stability.

Fig. 4 The calculated formation enthalpies (ΔH_form) of the doped hydrides with different Y concentrations of x = 0, 10%, 13%, 20%, 27%, 30%, 33% or 40%.

From the investigation of the substitution energy and the formation enthalpy above, Mg₈Ni₈Y₂H₃₆ is more energetically favorable than other co-doped compounds. Therefore, for the dehydrogenation property and bond and electronic structure of the Ni and Y co-doped system, we are going to investigate the Y doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆) for the co-doped systems, compared to the Ni single-doping case.

In order to further assess the dehydrogenation abilities of these hydrides, their hydrogen desorption enthalpies, ΔH_des, are calculated by using eqn (3):^42,43

ΔH_des = [E_t(Mg_10−nNi₈Y_nH₃₅) + 1/2E(H₂)] − E_t(Mg_10−nNi₈Y_nH₃₆) (3)

where E_t(Mg_18−nNi₈Y_nH₃₆) represents the total energy of hydrides. E_t(Mg_10−nNi₈Y_nH₃₅) refers to a pseudostructure in which one H atom is removed from the relaxed E_t(Mg_18−nNi₈Y_nH₃₆) system. E(H₂) is the same as that used in eqn (2).

The obtained hydrogen desorption enthalpy (ΔH_des) in single-doped MgH₂ with Ni is 0.71 eV. This enthalpy value is lower than the result in ref. 10, which may be ascribed to the higher dopant content here. Moreover, compared to ΔH_des = 1.5335 eV for pure MgH₂,^44,45 the hydrogen desorption enthalpy of the single-doped MgH₂ system is decreased. After Ni doping, the lower hydrogen desorption enthalpy of the doped MgH₂ system leads to an improvement of its dehydrogenation properties. For the best Ni and Y co-doped MgH₂ system, the hydrogen desorption enthalpies when moving out one H atom from each of the six non-equivalent positions of the co-doped system are all calculated. The obtained hydrogen desorption enthalpies (ΔH_des) are presented in Fig. 5. It can be observed that the hydrogen desorption enthalpies of the hydride are all significantly decreased in comparison with the Ni single-doped and pure MgH₂ systems. Thus, each hydrogen desorption is much easier than in the single-doped case, which indicates that the Ni and Y co-doped system exhibits excellent dehydrogenation properties. Besides, some of the hydrogen desorption enthalpies of the Ni and Y co-doped MgH₂ system are lower than those obtained when alloying MgH₂ with other catalysts.^21,24,28 Based on the calculated results we can conclude that co-doping MgH₂ with Ni and Y not only enhances the structural stability of MgH₂, but is also beneficial for the improvement of the dehydrogenation properties of MgH₂. Although the partial substitution of Mg by Ni and Y has significant synergetic effects on MgH₂, a detailed understanding of the influence of dopants on the hydrogen desorption process and kinetics of MgH₂ requires further investigation, which will be the subject of our future work.

Fig. 5 The calculated hydrogen desorption enthalpies (ΔH_des) when moving out one H atom (H1–H6) for the Y doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆) in (Mg, Ni)H₂.

3.3. Bonding analysis

Table 1 lists the bond distances between metallic elements in undoped and doped MgH₂. For undoped MgH₂, the bond length of Mg–Mg ranges from 3.50 Å to 4.48 Å with an average length of 3.66 Å. For the Ni single-doped MgH₂ system, Mg2 and Mg4 are the nearest neighboring metallic atoms of the dopant Ni and the Mg–Ni bonds are decreased by an average of 3.90 Å. Discounting the radius difference, the bond length of Mg–Ni is longer than the original Mg–Mg, which implies that the Mg–Ni bond is weakened. But the bond length of Mg–Mg is decreased, thus the strength of the Mg–Mg bond is enhanced compared with pure MgH₂. For the best co-doped case of Mg₈Ni₈Y₂H₃₆, discounting the radius difference, the distances between the Mg and Ni atoms are decreased in comparison with that of the single-doped system, which indicates that the bond strengths are strengthened. Furthermore, the lengths of the Mg–Y bond and the Y–Ni bond are shorter than the Ni-doped system, thus the bond strengths of Mg–Y and Y–Ni are enhanced. Thus it can be inferred that the dopant Y has a strong alloying trend with the Mg and Ni atoms, which weakens the strength of the Mg–H and Ni–H bonds.

Table 1 Calculated bond distances (Å) between metallic elements in the hydrides

MgH2	Mg1–Mg2	Mg1–Mg3	Mg1–Mg4	Mg2–Mg3	Mg2–Mg4	Mg3–Mg4
	3.50	4.48	3.50	3.50	3.50	3.50
Mg10Ni8H36	Mg1–Mg2	Mg1–Ni	Mg1–Mg4	Mg2–Ni	Mg2–Mg4	Ni–Mg4
	3.28	4.34	3.28	3.46	3.21	3.31
Mg8Ni8Y2H36	Mg1–Mg2	Mg1–Ni	Mg1–Mg4	Mg2–Ni	Mg2–Mg4	Ni–Mg4
	3.87	3.81	3.25	3.07	3.13	3.04
	Mg1–Y			Y–Ni	Y–Mg4
	3.36			3.09	3.40

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Table 2 lists the interaction of metallic and H atoms in undoped and doped MgH₂. In this calculation, the bond length of Mg–H ranges from 1.93 Å to 1.94 Å with an average length of 1.93 Å, consistent with the experimental³⁴ and other theoretical results.^35,36 For the Ni single-doped MgH₂ system, H4, H5 and H6 atoms are the nearest neighboring atoms of the dopant Ni and these atoms move towards Ni, thus the bond lengths of Mg–H (H4–H6) are increased. Meanwhile, the Mg–H (H5, H6) bond lengths are also increased. Hence, the bond strength of Mg–H is weakened. For Ni and Y co-doped MgH₂ (Mg₈Ni₈Y₂H₃₆), the nearest neighboring H (H4–H5) atoms of Ni further move towards the Ni, thus the Mg–H (H4–H5) bonds further weaken their strength. However the Ni–H6 bond is a little bit weakened. H1, H3 and H6 atoms are the nearest neighboring atoms of the dopant Y, in comparison with the single-doped system the bond length of Y–H (H1–H6) is markedly increased. The length of the Mg–H bonds are all increased, which means that the strength of the Mg–H bonds are all weakened. Therefore, the dopant Y can further weaken the Mg–H bond.

Table 2 Calculated bond distances (Å) between the metallic and H atoms in the hydrides

Mg10Ni8H36	Mg1–H1 1.80	Mg2–H1 1.84	Ni–H4-1 1.78	Mg4–H2 1.79
	Mg1–H2 1.84	Mg2–H3 1.80	Ni–H4-2 1.62	Mg4–H3 1.77
		Mg2–H6 2.25	Ni–H5 1.66	Mg4–H5 1.95
		Mg2–H1 3.46	Ni–H6 1.68
Mg8Ni8Y2H36	Mg1–H1 1.83	Mg2–H3 1.88	Ni–H4-1 1.72	Mg4–H2 2.00
	Mg1–H2 1.91	Mg2–H6 2.30	Ni–H4-2 1.62	Mg4–H3 1.83
		Y–H1 2.16	Ni–H5 1.59	Mg4–H5 1.99
		Y–H3 2.11	Ni–H6 1.70
		Y–H6 2.45

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3.4. Electronic structure

Fig. 6(a) shows the total and partial density of states (DOS) of undoped MgH₂. The Fermi energy (E_F) level is set at zero. The gap between the valence band (VB) and conduction band (CB) is about 4 eV, in good agreement with other calculation results.^28,46 The relatively large gap value of the bulk leads to a relatively high formation energy. The VB is mainly dominated by H s states and the CB mainly by Mg s and Mg p states. Analyzing the partial DOS curves, these correspond to the strong ionic bonding interactions between Mg and H atoms. By using the Bader charge analysis in molecule AIM theory,^47–49 it is found that the average electron number of the Mg and H atoms is 0.42 and 1.79, respectively, as shown in Table 3. The ionic charges of Mg and H can be represented as Mg^1.58+ and H^0.79−, indicating the strong ionic character of the Mg–H bond.

Fig. 6 Total and partial densities of states (DOS) of (a) undoped MgH₂; (b) Mg₁₀Ni₈H₃₆; (c) Mg₈Ni₈Y₂H₃₆.

Table 3 Bader charges for undoped and doped MgH₂ systems

MgH2	Mg1	Mg2	Mg3	Mg4
	0.43	0.42	0.42	0.42
	H1	H2	H3	H4	H5	H6
	1.79	1.79	1.79	1.80	1.79	1.79
Mg10Ni8H36	Mg1	Mg2	Ni	Mg4
	0.44	0.41	9.63	0.43
	H1	H2	H3	H4	H5	H6
	1.79	1.78	1.81	1.22	1.61	1.32
Mg8Ni8Y2H36	Mg1	Mg2	Y	Ni	Mg4
	0.44	0.39	9.33	9.66	0.43
	H1	H2	H3	H4	H5	H6
	1.76	1.79	1.75	1.23	1.62	1.33

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Fig. 6(b) shows the total and partial DOS of the Ni single-doped MgH₂ system. In order to plot the partial DOS of all atoms in the same panel with the same scale, the amplitude of the partial DOS of the Ni d orbital was decreased by 10. The total DOS curve shows a remarkable decrease in the band gap of 0 eV, which show clearly metallic characteristics. It can be obviously seen that the Ni d states insert in the middle part and then overlap with the Mg p and H s orbitals separately in a different energy region. The electrons in the Mg p and H s hybridization states are pushed to the Mg p, Ni d and H s hybridization states. The interactions of the Mg–H bond in these regions are weakened. Furthermore, the H s orbitals distributed in the region of −5.5 to −3.1 eV are less overlapped with the Mg s and p orbitals compared to the pure MgH₂, which can help to weaken the hybridization of the Mg–H bond. This interpretation is also supported by the Bader charge data. As shown in Table 3, the Bader charges on H (H4–H6) atoms decrease significantly and the Bader charges on Mg (Mg1, Mg4) increase slightly after Ni-doping. Therefore, the electron transfer from H to Mg is obviously weakened.

Fig. 6(c) shows the total and partial DOS of the Ni and Y co-doped MgH₂ system; this case corresponds to a Y doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆). In order to plot the partial DOS of all atoms in the same panel with the same scale, the amplitude of the partial DOS of Ni d and Y d orbitals were decreased by 10 and 5, respectively. The main peaks of the co-doped system slightly move away from the Fermi energy compared with the Ni single-doped MgH₂, implying a higher stability of the Ni and Y co-doped system. The Y p and d orbitals overlap with H s, Mg p, and Ni p and d orbitals in the energy region of −6.0 to −2.6 eV. Also, the distributions of the bonding peaks of Y d orbitals are mainly concentrated in the energy region of 1.2 to 5.1 eV and overlap well with the Mg s and p orbitals. These behaviors indicate that the Y atom has a strong bonding interaction with H, Mg and Ni atoms and decreases the Mg–H p–s mixing. In addition, the drop in magnitude of the bonding peaks of H s and Mg s and p orbitals is more than that of the Ni single-doping, which means that the Mg–H bond is further weakened in the co-doped system. Similar to the Ni single-doped system, this interpretation is also supported by the Bader charge data. The number of electrons around H (H1, H3) atoms in the co-doped system is decreased compared with Ni single-doped MgH₂, which implies that the electron transfer from H to Mg is further weakened. Thus, the Mg–H hybridizations are significantly weakened by Ni and Y co-doping.

4. Conclusion

In summary, first-principles calculations were preformed to study the structural stabilities and dehydrogenation properties of destabilized MgH₂ co-doped with Ni and Y. According to the minimal total electronic energy, the Ni and Y prefer to substitute in the Mg3 and Mg2 position, respectively. The substitution energies and formation enthalpies of different Y doping concentrations of hydrides were estimated. They show that most of the Ni and Y co-doped MgH₂ systems are more stable than Ni single-doping. Especially, the Y doping concentration of x = 20% (Mg₈Ni₈Y₂H₃₆) has the most energetic stability. Moreover, these calculations also give insight into the hydrogen desorption enthalpies (ΔH_des) of this cases. The Ni and Y co-doped MgH₂ system has the lowest ΔH_des value of 0.11 eV. Due to the relatively lower hydrogen desorption enthalpy, the Ni and Y co-doped MgH₂ system possesses promising dehydrogenation properties. The electronic structures show that the hybridization of dopants with Mg and H atoms together weaken the Mg–H interaction. The electronic structures further demonstrate that the Mg–H bonds are more susceptible to dissociation by Ni and Y co-doping because of the reduced magnitude of the Mg–H hybridization peaks. Therefore, co-doping with Ni and Y effectively improves the dehydrogenation properties of destabilized MgH₂.

Acknowledgements

The work was supported by the National Natural Science Fund (No. 11504228) and the Graduated Innovative Research Project of Shanghai University of Engineering Science (No. E1-0903-14-01107-14KY0411).

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