Improvement in the hydrogen desorption from MgH2 upon transition metals doping: Ahybrid density functional calculationsTanveer Hussain, Tuhina Adit Maark, Biswarup Pathak, and Rajeev Ahuja Citation: AIP Advances 3, 102117 (2013); doi: 10.1063/1.4826521 View online: http://dx.doi.org/10.1063/1.4826521 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/3/10?ver=pdfcov Published by the AIP Publishing

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AIP ADVANCES 3, 102117 (2013)

Improvement in the hydrogen desorption from MgH2 upontransition metals doping: A hybrid density functionalcalculations

Tanveer Hussain,1,a Tuhina Adit Maark,2 Biswarup Pathak,4

and Rajeev Ahuja1,3

1Condensed Matter Theory Group, Department of Physics and Astronomy UppsalaUniversity, S-75120 Uppsala, Sweden2School of Engineering, Brown University, Providence, RI 02912, USA3Department of Materials and Engineering, Royal Institute of Technology (KTH),S-100 44 Stockholm, Sweden4Discipline of Chemistry, Indian Institute of Technology - Indore, 452017, India

(Received 1 August 2013; accepted 9 October 2013; published online 18 October 2013)

This study deals with the investigations of structural, electronic and thermodynamicproperties of MgH2 doped with selected transition metals (TMs) by means of hy-brid density functional theory (PBE0). On the structural side, the calculated latticeparameters and equilibrium volumes increase in case of Sc, Zr and Y opposite toall the other dopants indicating volumetrically increased hydrogen density. ExceptFe, all the dopants improve the kinetics of MgH2 by reducing the heat of adsorptionwith Cu, Nb, Ni and V proving more efficient than others studied TM’s. The elec-tronic properties have been studied by density of states and correlated with hydrogenadsorption energies. C© 2013 Author(s). All article content, except where otherwisenoted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4826521]

High carbon dioxide emissions from the excess use of the fossil fuels have caused significantharm to the environment and human lives.1, 2 Finding a cheap, clean and efficient alternative to theavailable energy resources is of prime importance today.

In this regard hydrogen has tremendous potential with its high-energy content and possibility ofzero emissions when used in fuel cells.3, 4 However, the storage of hydrogen has been a challengingtask on account of its low energy density. Materials based solid-state storage provides a safer, costeffective and less technically demanding option than the conventional methods (gaseous storage andliquefaction). 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.5

MgH2 satisfies the first criterion having a high hydrogen capacity of 7.5 wt% but not the other two.One of the routes that have been considered to overcome these limitations is the addition of dopantsand additives to MgH2.6–8

Recently, Milosevic et al.9 employed the mechanical milling of NaNH2 to improve the desorptionproperties of MgH2. It was revealed that with 2 wt% of NaNH2 and short milling time (∼15min) thebest desorption efficiency could be achieved. Li et al.10 have reported that the milling of MnFe2O4

with MgH2 caused a dramatic improvement in its dehydrogenation characteristics coupled with asignificant 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 ofMgH2. They concluded that the defects formed due to the reaction of MgH2 with Ti and Ni on thesurface/interior and the formation of Mg2Ni caused the improvement in its hydriding/dehydridingproperties.

atanveer.hussain@physics.uu.se

2158-3226/2013/3(10)/102117/8 C© Author(s) 20133, 102117-1

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102117-2 Hussain et al. AIP Advances 3, 102117 (2013)

In order to develop an insight on the effect of additives on MgH2, it is important to gain afundamental understanding of the underlying processes via theoretical studies.12–16 Zeng et al.17

reported the destabilization effects of MgH2 by doping with 3d transition metals. By means of firstprinciples calculation, the order of destabilization caused by selected dopants and nature of bondingin pure as well as doped systems was revealed. Shelyapina et al.18 investigated the effect of mixingof Tm in MgH2 to accelerate the kinetics of hydrogen. Based on total energy calculations, theyestimated the heat of formation and relative stabilities of newly synthesized Ca7Ge type hydridesMg6MH16 (M = Ti, V, and Nb) as compared to MgH2. All of the hydrides were found to beless stable than pure MgH2. The stability of these hydrides was further reduced by the formationof Mg vacancy. The doping effects of light transition metals (Sc, Ti, V, Cr) on the stability andthermodynamics of MgH2 were studied by Er. et al.19 Based on first principles density functionalcalculations a phase transition from fluorite to rutile was observed upon doping of selected Tm.Some others studies20, 21 also described the influence of the dopants on the kinetics of MgH2 andcoupled the destabilization of the systems with the reduction of the band gap. However the band gapcalculations were performed by GGA approximation, which is well know for its underestimationdue 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 thisband gap problem. Recently, by a DFT study of α-, γ -, and β-MgH2 we showed that the PBE022, 23

calculated band gaps of the pure phases were in excellent agreement with previously reported GWvalues. In addition the predicted band gaps due to Al and Si doping were also larger than basedon GGA-PBE suggesting semiconducting and insulating properties, respectively, as opposed to theearlier belief of transition to metallic and semiconducting behavior.24

Motivated by this we herein investigate the electronic, thermodynamic and optical properties ofTM (Sc, V, Fe, Co, Ni, Cu, Y, Zr, Nb) doped -MgH2 employing the PBE0 hybrid density functional.

All the calculations were performed using DFT25 based Vienna ab initio simulations package(VASP).26, 27 The approximation used in this study are the PBE approach utilizing a standard GGAscheme called GGA-PBE28 and the hybrid density functional PBE022, 23 Projector augment waves(PAWs) and plan-wave (PW) basis set as provided with VASP have been employed. The forcesand 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-offenergy of 440 eV was chosen for the PW basis set. The convergence of the structural parame-ters and the band gap calculation was tested against 7 × 7 × 3 k-points grid.29 A 1 × 1 × 3super cell of MgH2 have 18 atoms (Mg = 6, H = 12) was used in this study. For studying theeffect of TM doping, one Mg atom was substituted with TM atom resulting into a doping con-centration of 16.66%. The optimized structure of pure and Cu doped MgH2 has been shown inFigure 1(a) and 1(b).

First of all, structural properties are examined in terms of the lattice parameters and equilibriumvolumes of all the systems and presented in Table I. It is clear from the table that a, b, c valuesand equilibrium volumes increase for Sc, Zr and Y doping and decrease for all other TMs. Thepercentage reduction in volume caused is within a range of −2.4% to −9.2%. This in turn wouldlead to enhance volumetric hydrogen storage density in these TM (except Sc, Zr and Y) dopedmagnesium hydrides. The lattice parameters calculated in this study agree reasonably well with theprevious study.10, 30 The above changes also lead to slight differences in the atomic arrangement.In case of pure α-Mg6H12 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-MgH2

is displayed in Fig. 1(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 bondlengths 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. Theelongation of Mg-H bond lengths is suggestive of an easier dissociation and consequently reducedhydrogen adsorption energy. In case of the elements Sc, Zr and Y, which caused the increment inthe volume the Mg-Mg and Mg-H distances are calculated as (3.49 Å, 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 otherTM’s are the indication of slightly higher adsorption energies.

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102117-3 Hussain et al. AIP Advances 3, 102117 (2013)

FIG. 1. Optimized structures of (a) pure and (b) Cu-doped MgH2. Orange, green and red balls represent Mg. H and Cu atomsrespectively.

TABLE I. Lattice parameters (a, b, c) and the equilibrium volumes (V) of different transition metals doped α-MgH2 calculatedby hybrid functional PBE0 approach is given. Where n = 6 for all the systems.

System a (Å) b (Å) c (Å) V (Å3/n)

Mg5H12Sc 4.4819 4.4819 3.0650 30.81Mg5H12Ni 4.4216 4.4216 2.8999 28.34Mg5H12V 4.3776 4.3776 2.9962 28.71Mg5H12Fe 4.3692 4.3692 2.8874 27.56Mg5H12Co 4.3881 4.3881 2.8893 27.80Mg5H12Cu 4.4044 4.4044 2.9706 28.81Mg5H12Y 4.5077 4.5077 3.1640 32.13Mg5H12Zr 4.4329 4.4329 3.1120 30.58Mg5H12Nb 4.3924 4.3924 3.0706 29.62

Next, we discuss and report the effect of dopants in reducing the heat of formations by calculatingthe total energies of the pure as well as TM doped MgH2 systems. The following relation has beenused to calculate the adsorption energies in order to investigate the effect of alloying element (TM’s)on the stability of the system:

Eads = {n E (MgH2 : X) − (n − 1) E (Mg) − n E (H2) − E (X)}/n (1)

Where E (MgH2: X), E (Mg) and E (H2) are the total energies of the TM doped MgH2, Mg and H2

molecules. Here X = Sc, V, Fe, Co, Ni, Cu, Y, Zr and Nb and ‘n’ is the number of formula units thatis six in this case. E (X) is the energy of the each TM metal adatoms considered here. The energy ofH2 molecules has been calculated by putting the H2 molecule in a cell of 15 × 15 × 15 dimensions.The calculated values of Eads in case of pure as well as doped systems have been presented inTable II. The experimental and theoretical values of the adsorption energies of hydrogen for pureMgH2 has been reported to be in the range of −0.682 eV to −0.772 eV31 The general trend followedby the TM’s and their beneficial effect in reducing Eads has been found as Cu > Nb > Ni > V > Zr> Co > Sc > Y. By looking at this trend, it can be stated that the dehydrogenation thermodynamicshas improved much more significantly with the dopants from late 3d transition metals as comparedto the early ones and to the 4d transition metals. The only substituent, which does not improve the

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102117-4 Hussain et al. AIP Advances 3, 102117 (2013)

TABLE II. The heat of adsorption �Hads (eV/H2) of TM doped MgH2 systems calculated by PBE0 functional.

System �Hads

Mg5H12Sc −0.6449Mg5H12V −0.4193Mg5H12Fe −0.8656Mg5H12Co −0.6419Mg5H12Ni −0.3979Mg5H12Cu −0.1431Mg5H12Y −0.6535Mg5H12Zr −0.4801Mg5H12Nb −0.3207

FIG. 2. Total and partial density of states of pure (Mg6H12) calculated by PBE0 functional.

thermodynamics by decreasing Eads, is Fe, which can be seen by the high value of adsorption energyin Table II. Cu is the best dopant bringing out a ∼1/5th reduction Eads of MgH2. Though the valueof −0.1431 eV is weaker than the range of −20 to −40 kJ/mol H2 considered ideal for hydrogenstorage at temperatures between −30oC to 50oC32 using a lower fraction of doping than in this workwould help serve the purpose. In comparison Eads for MgH2-Nb is well within the binding energyrange and of MgH2-Ni and -V are just on the borderlines making them suitable substituents as wellfor improving the thermodynamics of the system. This trend is consistent with the previous study11

where the destabilization occurring in MgH2 system by alloying TM substituents was attributedto the possible formation of some complex Mg-TM or H-TM compounds. Complex hydrides likeMgCu2, Mg2NiH4 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 MgH2 and selected TM doped-MgH2. From Fig. 2 it can be seen that in Mg6H12 the bonding peaks between the Fermi level (EF)and −6.0 eV are due to the interaction between H and Mg electrons. The high DOS intensity at EF is

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102117-5 Hussain et al. AIP Advances 3, 102117 (2013)

FIG. 3. Total and partial density of states of (Mg5H12Cu).

indicative of a strong bonding. The valence band (VB) is separated by an energy gap of 5.4 eV fromthe conduction band (CB) and consequently MgH2 has high formation energy. This picture changesin MgH2-Cu. Comparing Figs. 2 and 3 we observe that the original VB of pure-MgH2 is shifteddown in energy in MgH2-Cu and additional peaks are formed around the EF between −2.0 to 1.0 eV.This effectively reduces the band gap to zero, which should allow for easy mobility of electrons intothe 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. Thepartial DOS in Fig. 3 shows that the above- mentioned additional states are primarily belong to Cuand have some contribution from Mg and H states.

In case of MgH2-Nb, MgH2-Ni and MgH2-V from both the total and partial DOS (seeFigs. 4–6) it is evident that new states arising predominantly from the dopants are generated be-tween the original band gaps of pure MgH2 and are positioned just below the EF between −1.0 and0.0 eV. There is a clear band gap of 1.87 eV 2.0 eV and 3.25 eV for MgH2-Nb, MgH2-Ni andMgH2-V, respectively. These values are much less than that of pure MgH2 but greater than MgH2-Cu, which suggests that Mg-H bonds in these systems will be more susceptible to dissociation thanMgH2 and less susceptible than MgH2-Cu. The above order of band gaps reflects the trend observedin Eads values.

In conclusion, in this study we have studied TMs (Sc, V, Fe, Co, Ni, Cu, Y, Zr, Nb) dopedα-MgH2 using the hybrid density functional PBE0. Except for Sc, Zr and Y substitution of Mgwith TMs caused a decrease in the equilibrium volume, implying an increased volumetric hydrogenstorage density in these systems. Addition of TMs, barring Fe, resulted in destabilization of MgH2 asexemplified by the reduced (i.e. less negative) heats of adsorption. Cu, Nb, Ni and V were predictedto be most promising dopants for enhancing the dehydrogenation thermodynamics of MgH2 andthereby, improve its hydrogen storage characteristics. The trends in hydrogen adsorption energieswere explained in terms of the reduced band gaps.

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102117-6 Hussain et al. AIP Advances 3, 102117 (2013)

FIG. 4. Total and partial density of states of (Mg5H12Nb).

FIG. 5. Total and partial density of states of (Mg5H12Ni).

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102117-7 Hussain et al. AIP Advances 3, 102117 (2013)

FIG. 6. Total and partial density of states of (Mg5H12V).

ACKNOWLEDGMENT

TH is thankful to higher education commission of Pakistan for doctoral fellowship. RA acknowl-edges FORMAS, SWECO and Wenner-Gren Foundation for financial support. SNIC and UPPMAXare acknowledged for computing time. Leading Foreign Research Institute Recruitment Programthrough 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.

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