Improvement in the hydrogen desorption from MgH 2 upon transition metals doping : A hybrid density functional calculations

This study deals with the investigations of structural, electronic and thermodynamic properties of MgH2 doped with selected transition metals (TMs) by means of hybrid density functional theory (PBE0). On the structural side, the calculated lattice parameters and equilibrium volumes increase in case of Sc, Zr and Y opposite to all the other dopants indicating volumetrically increased hydrogen density. Except Fe, all the dopants improve the kinetics of MgH2 by reducing the heat of adsorption with Cu, Nb, Ni and V proving more efficient than others studied TM’s. The electronic properties have been studied by density of states and correlated with hydrogen adsorption energies.

High carbon dioxide emissions from the excess use of the fossil fuels have caused significant harm to the environment and human lives. 1,2 inding a cheap, clean and efficient alternative to the available energy resources is of prime importance today.
In this regard hydrogen has tremendous potential with its high-energy content and possibility of zero emissions when used in fuel cells. 3,4 owever, the storage of hydrogen has been a challenging task on account of its low energy density.Materials based solid-state storage provides a safer, cost effective and less technically demanding option than the conventional methods (gaseous storage and liquefaction).Such materials must store high hydrogen by weight (5.5wt%) and volume (0.04Kg/L), be operational at moderate temperature-pressure conditions and store/release hydrogen quickly. 5gH 2 satisfies the first criterion having a high hydrogen capacity of 7.5 wt% but not the other two.][8] Recently, Milosevic et al. 9 employed the mechanical milling of NaNH 2 to improve the desorption properties of MgH 2 .It was revealed that with 2 wt% of NaNH 2 and short milling time (∼15min) the best desorption efficiency could be achieved.3][14][15][16] Zeng et al. 17 reported the destabilization effects of MgH 2 by doping with 3d transition metals.By means of first principles calculation, the order of destabilization caused by selected dopants and nature of bonding in pure as well as doped systems was revealed.Shelyapina et al. 18 investigated the effect of mixing of Tm in MgH 2 to accelerate the kinetics of hydrogen.Based on total energy calculations, they estimated the heat of formation and relative stabilities of newly synthesized Ca 7 Ge type hydrides Mg 6 MH 16 (M = Ti, V, and Nb) as compared to MgH 2 .All of the hydrides were found to be less stable than pure MgH 2 .The stability of these hydrides was further reduced by the formation of Mg vacancy.The doping effects of light transition metals (Sc, Ti, V, Cr) on the stability and thermodynamics of MgH 2 were studied by Er. et al. 19 Based on first principles density functional calculations a phase transition from fluorite to rutile was observed upon doping of selected Tm.Some others studies 20,21 also described the influence of the dopants on the kinetics of MgH 2 and coupled the destabilization of the systems with the reduction of the band gap.However the band gap calculations were performed by GGA approximation, which is well know for its underestimation due to the discontinuity of exchange and correlation.
Hybrid functionals, which are a combination of an exact nonlocal orbital-dependent Hatree-Fock (HF) exchange and a standard local exchange-correlation functional, are known to address this band gap problem.Recently, by a DFT study of α-, γ -, and β-MgH 2 we showed that the PBE0 22,23 calculated band gaps of the pure phases were in excellent agreement with previously reported GW values.In addition the predicted band gaps due to Al and Si doping were also larger than based on GGA-PBE suggesting semiconducting and insulating properties, respectively, as opposed to the earlier belief of transition to metallic and semiconducting behavior. 24otivated by this we herein investigate the electronic, thermodynamic and optical properties of TM (Sc, V, Fe, Co, Ni, Cu, Y, Zr, Nb) doped -MgH 2 employing the PBE0 hybrid density functional.
All the calculations were performed using DFT 25 based Vienna ab initio simulations package (VASP). 26,27 he approximation used in this study are the PBE approach utilizing a standard GGA scheme called GGA-PBE 28 and the hybrid density functional PBE0 22,23 Projector augment waves (PAWs) and plan-wave (PW) basis set as provided with VASP have been employed.The forces and stress minimization while relaxing the unit cell shape and volume optimized the geometries.Relaxation was performed until the force on each atom became less than 0.005 eV/Å.A cut-off energy of 440 eV was chosen for the PW basis set.The convergence of the structural parameters and the band gap calculation was tested against 7 × 7 × 3 k-points grid. 29A 1 × 1 × 3 super cell of MgH 2 have 18 atoms (Mg = 6, H = 12) was used in this study.For studying the effect of TM doping, one Mg atom was substituted with TM atom resulting into a doping concentration of 16.66%.The optimized structure of pure and Cu doped MgH 2 has been shown in Figure 1(a) and 1(b).
First of all, structural properties are examined in terms of the lattice parameters and equilibrium volumes of all the systems and presented in Table I.It is clear from the table that a, b, c values and equilibrium volumes increase for Sc, Zr and Y doping and decrease for all other TMs.The percentage reduction in volume caused is within a range of −2.4% to −9.2%.This in turn would lead to enhance volumetric hydrogen storage density in these TM (except Sc, Zr and Y) doped magnesium hydrides.The lattice parameters calculated in this study agree reasonably well with the previous study. 10,30 he above changes also lead to slight differences in the atomic arrangement.In case of pure α-Mg 6 H 12 the nearest Mg-Mg and Mg-H distances are 3.517 Å and 1.936 Å, respectively.As a representative example the optimized geometries geometry of Cu doped-MgH 2 is displayed in Fig. 1   Next, we discuss and report the effect of dopants in reducing the heat of formations by calculating the total energies of the pure as well as TM doped MgH 2 systems.The following relation has been used to calculate the adsorption energies in order to investigate the effect of alloying element (TM's) on the stability of the system: Where E (MgH 2 : X), E (Mg) and E (H 2 ) are the total energies of the TM doped MgH 2 , Mg and H 2 molecules.Here X = Sc, V, Fe, Co, Ni, Cu, Y, Zr and Nb and 'n' is the number of formula units that is six in this case.E (X) is the energy of the each TM metal adatoms considered here.The energy of H 2 molecules has been calculated by putting the H 2 molecule in a cell of 15 × 15 × 15 dimensions.The calculated values of E ads in case of pure as well as doped systems have been presented in Table II.The experimental and theoretical values of the adsorption energies of hydrogen for pure MgH 2 has been reported to be in the range of −0.682 eV to −0.772 eV 31 The general trend followed by the TM's and their beneficial effect in reducing E ads has been found as Cu By looking at this trend, it can be stated that the dehydrogenation thermodynamics has improved much more significantly with the dopants from late 3d transition metals as compared to the early ones and to the 4d transition metals.The only substituent, which does not improve the thermodynamics by decreasing E ads , is Fe, which can be seen by the high value of adsorption energy in Table II.Cu is the best dopant bringing out a ∼1/5 th reduction E ads of MgH 2 .Though the value of −0.1431 eV is weaker than the range of −20 to −40 kJ/mol H 2 considered ideal for hydrogen storage at temperatures between −30 o C to 50 o C 32 using a lower fraction of doping than in this work would help serve the purpose.In comparison E ads for MgH 2 -Nb is well within the binding energy range and of MgH 2 -Ni and -V are just on the borderlines making them suitable substituents as well for improving the thermodynamics of the system.This trend is consistent with the previous study 11 where the destabilization occurring in MgH 2 system by alloying TM substituents was attributed to the possible formation of some complex Mg-TM or H-TM compounds.Complex hydrides like MgCu 2 , Mg 2 NiH 4 etc. have been experimentally identified and reported by Song et al. 11 Finally, here we correlate the improved hydrogen adsorption energies to the electronic structure.To this end we analyze the total and partial density of DOS of pure MgH 2 and selected TM doped-MgH 2 .From Fig. 2 it can be seen that in Mg 6 H 12 the bonding peaks between the Fermi level (E F ) and −6.0 eV are due to the interaction between H and Mg electrons.The high DOS intensity at E F is This effectively reduces the band gap to zero, which should allow for easy mobility of electrons into the CB and soften the Mg-H bond leading to a reduction (less negative value) in formation energy.Furthermore, we expect a transition from insulating to metallic properties on doping with Cu.The partial DOS in Fig. 3 shows that the above-mentioned additional states are primarily belong to Cu and have some contribution from Mg and H states.
In case of MgH 2 -Nb, MgH 2 -Ni and MgH 2 -V from both the total and partial DOS (see Figs. 4-6) it is evident that new states arising predominantly from the dopants are generated between the original band gaps of pure MgH 2 and are positioned just below the E F between −1.0 and 0.0 eV.There is a clear band gap of 1.87 eV 2.0 eV and 3.25 eV for MgH 2 -Nb, MgH 2 -Ni and MgH 2 -V, respectively.These values are much less than that of pure MgH 2 but greater than MgH 2 -Cu, which suggests that Mg-H bonds in these systems will be more susceptible to dissociation than MgH 2 and less susceptible than MgH 2 -Cu.The above order of band gaps reflects the trend observed in E ads values.
In conclusion, in this study we have studied TMs (Sc, V, Fe, Co, Ni, Cu, Y, Zr, Nb) doped α-MgH 2 using the hybrid density functional PBE0.Except for Sc, Zr and Y substitution of Mg with TMs caused a decrease in the equilibrium volume, implying an increased volumetric hydrogen storage density in these systems.Addition of TMs, barring Fe, resulted in destabilization of MgH 2 as exemplified by the reduced (i.e. less negative) heats of adsorption.Cu, Nb, Ni and V were predicted to be most promising dopants for enhancing the dehydrogenation thermodynamics of MgH 2 and thereby, improve its hydrogen storage characteristics.The trends in hydrogen adsorption energies were explained in terms of the reduced band gaps.
have reported that the milling of MnFe 2 O 4 with MgH 2 caused a dramatic improvement in its dehydrogenation characteristics coupled with a significant reduction in desorption temperature of hydrogen.In another current study Song.et al. 11 investigated experimentally the influence of Ti and Ni on hydriding and dehydriding properties of MgH 2 .They concluded that the defects formed due to the reaction of MgH 2 with Ti and Ni on the surface/interior and the formation of Mg 2 Ni caused the improvement in its hydriding/dehydriding properties.a tanveer.hussain@physics.uu.se 2158-3226/2013/3(10)/102117/8 C Author(s) 2013 3, 102117-1 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license.See: http://creativecommons.org/licenses/by/3.0/Downloaded to IP: 130.238.171.100On: Tue, 10 Dec 2013 08:01:18 (b).The modified closest Mg-Mg and Mg-H distances in case of V (3.53 Å, 1.95 Å), Ni (3.56 Å, 1.94 Å), Cu (3.55 Å, 1.99 Å) and Nb (3.52 Å, 1.93 Å).The respective bond lengths of V-Mg, Ni-Mg, Cu-Mg and Nb-Mg are found to be 2.97 Å, 2.90 Å, 2.93 Å and 3.02 Å.The elongation of Mg-H bond lengths is suggestive of an easier dissociation and consequently reduced hydrogen adsorption energy.In case of the elements Sc, Zr and Y, which caused the increment in the volume the Mg-Mg and Mg-H distances are calculated as (3.49Å, 1.91 Å), (3.50 Å, 1.93 Å) and (3.478 Å, 1.89 Å) respectively.Relatively shorter Mg-H bond lengths in comparison to the other TM's are the indication of slightly higher adsorption energies.

FIG. 1 .
FIG. 1. Optimized structures of (a) pure and (b) Cu-doped MgH 2 .Orange, green and red balls represent Mg.H and Cu atoms respectively.TABLE I. Lattice parameters (a, b, c) and the equilibrium volumes (V) of different transition metals doped α-MgH 2 calculated by hybrid functional PBE0 approach is given.Where n = 6 for all the systems.System a (Å) b (Å) c (Å) V (Å 3 /n)

ACKNOWLEDGMENT
TH is thankful to higher education commission of Pakistan for doctoral fellowship.RA acknowledges FORMAS, SWECO and Wenner-Gren Foundation for financial support.SNIC and UPPMAX are acknowledged for computing time.Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (MEST) (Nos.2010-00218 and 2010-0000751), supported this work.

TABLE II .
The heat of adsorption H ads (eV/H 2 ) of TM doped MgH 2 systems calculated by PBE0 functional.
FIG. 2. Total and partial density of states of pure (Mg 6 H 12 ) calculated by PBE0 functional.