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High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement
international journal of hydrogen energy xxx (xxxx) xxx
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High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement Prayoonsak Pluengphon a,*, Thiti Bovornratanaraks b,c, Prutthipong Tsuppayakorn-aek b,c, Udomsilp Pinsook b,c, Burapat Inceesungvorn d,** a
Division of Physical Science, Faculty of Science and Technology, Huachiew Chalermprakiet University, Samutprakarn, 10540, Thailand b Extreme Conditions Physics Research Laboratory (ECPRL) and Physics of Energy Materials Research Unit (PEMRU), Department of Physics, Faculty of Science, Chulalongkorn University, 10330, Bangkok, Thailand c Thailand Center of Excellence in Physics, Commission on Higher Education, 328 Si-Ayuttaya Road, Bangkok, 10400, Thailand d Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Center of Excellence in Materials Science and Technology, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand
highlights
graphical abstract
The g-phase has possessed the minimum free energy in the TMdoped systems. The
occupation
of
TM
sub-
stitutions in the b and g phases is easier than in the a phase. High-pressure induces the occupation
of
H-vacancy
into
the
compounds. The activation energy curves of Ni-, Pd- and Pt-doped MgH2 under pressure are presented. The minimum activation energy barriers are significantly dominated in the g phase.
article info
abstract
Article history:
The improvement of the hydrogen storage mechanism in TM-doped MgH2 by structural
Received 2 May 2019
high-pressure effects has been found using ab initio calculation. Phase transition, formation
Received in revised form
enthalpy and H-vacancy mechanism of a-, b-, and g-MgH2 with 3.125% of Ni, Pd and Pd
28 May 2019
dopants are analyzed under the pressure conditions of 0, 5 and 10 GPa. It is found that the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Pluengphon), [email protected] (B. Inceesungvorn). https://doi.org/10.1016/j.ijhydene.2019.06.066 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
2 Accepted 11 June 2019 Available online xxx
international journal of hydrogen energy xxx (xxxx) xxx
enthalpy of b- and g-phases based on the a-phase decreases in TM-doped systems, especially for the heavier atomic size of dopants (Pt > Pd > Ni). As a result, the g-phase has become structural stable at ambient pressure. The occupation enthalpy of TM substitutions
Keywords:
in b and g phases is easier than that in the a phase, which indicates ability of mixing
Doped-MgH2
impurities. High pressure induces the occupation of H-vacancy in all compounds. The
H-vacancy
activation energy curves of MgH2 with Ni, Pd and Pt dopants are also analyzed, and the
Kinetics diffusion
minimal barriers are significantly dominated in the g phase.
Hydrogen storage
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High pressure
Introduction Hydrogen energy is one of the most vivid solutions for finding a future fuel source, especially in vehicle technology, due to its nontoxic gas emission and unlimited source on the earth. Using a hydrogen energy carrier by directly containing gas in a tank is at a too low efficiency level for burning fuel for a long time in mobile vehicles [1e4]. Therefore, hydrogen storage methods have been widely studied for enhancing the efficiency of the energy carrier and internal process [1,3]. Solid state compounds which are used in the hydrogen storage method transfer hydrogen gas through chemical methods. A hydrogen storage process with a metal hydride group [5,6] has been initially presented as a new way to store hydrogen energy, and it has the special capability to reversibly store and release hydrogen [3,5]. Magnesium hydride (MgH2) is one of the candidate practical materials used in on-board hydrogen storage in vehicles because of its low cost and high gravimetric and volumetric capacities of H2 (7.7 wt% and 109 gH2) [5]. However, pure MgH2 gives slow reaction kinetics and a high temperature of absorption/desorption processes. In the past, crystal structures and phase transitions of MgH2 under high pressure were intensively studied in both theory and experiment investigations [7e9]. By investigation of X-ray powder and neutron diffractions [7], it was found that the ambient phase aMgH2 transforms to an g-MgH2 orthorhombic phase at the pressure 2 GPa. The theoretical study based on density functional theory (DFT) [8] presented the existence of other highpressure phases that are a; b and g structures. Later, the DFT result was verified by a high-pressure synchrotron X-ray diffraction technique [9]. The reaction enthalpy of pure MgH2 at 0 K was reported as 64.7 kJ/mol H2 [10]. A large value of heat in the adsorption-desorption process is a main problem for improvement. There is recent progress in tuning of diffusion kinetics or thermodynamic properties by additive compositions [11,12]. Improvement of H2 adsorption-desorption in MgH2 has been widely introduced by adding a transition metal (TM) as an additive or dopant [13e22]. The desorption temperature and dehydrogenation enthalpy of the MgeNb2O5 mixtures were lower than that of pure MgH2 [13]. The free energies with Fe catalysts indicated the lower desorption temperature [14]. Liang et al. [15] studied ball-milled MgH2-TM nanocomposites with Ti, V, Mn, Fe and Ni. They showed that the activation energy of desorption for MgH2 was significantly decreased by milling with TM. Hanada et al. [16] catalyzed a
MgH2 composite by milling with a TM-nanoparticle of Fe, Co, Ni, and Cu. They found that Ni doping gave the minimum activation energy for hydrogen desorption. Kodera et al. [17] studied the Mg2NiH4 compound using the mechanical grinding process and found that it decreases absorption pressures and enhances the reaction kinetics. Later, the DFT investigations of TM-doped a-, g-, and b-MgH2 were analyzed [18,19]. The Ni dopant significantly decreases the lattice constants. The TM dopants influence the stability of both b- and gMgH2, and they can decrease the hydrogen desorption from band g-MgH2 at much lower temperatures than from pure MgH2 [19]. German and Gabauer [20] presented the improvement of hydrogen vacancy diffusion kinetics in a-MgH2 by Nb and Zr dopants, which was determined from activation energy curves. Moreover, the PdeNi bimetallic dopants were synthesized by an experiment with mesoporous carbon [21] that presented enhanced dehydrogenation performance in MgH2. Doping effect on the layers of MgH2 (110) surface was observed [22] and found that the impurity at the first and second layers presented the lowering of H2 desorption. In this study, we found that the diffusion kinetics of Hvacancy in TM-doped MgH2 at high-pressure phases remains incomplete. Therefore, this report is focused on the hydrogen diffusion kinetics mechanisms in the three first phases (a; b and g) of MgH2 with Ni, Pd and Pt dopants. Structural phase transition, formation enthalpy, occurrence of H-vacancy enthalpy and activation energy curves are intensively investigated using ab initio study. Our calculation results indicate that high-pressure structures induce the better solutions for improving of the mechanism of hydrogen desorption and the energy barrier of H-vacancy diffusion kinetics.
Calculation details In this research, an ab initio calculation based on DFT was performed to solve Kohn-Sham equations [23] as implement in Cambridge Serial Total Energy Package (CASTEP) [24]. The minimum free energy, effective potential and electron density at a ground state condition were evaluated by self-consistent field method [23]. The generalized-gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) [25] was selected as the exchange-correlation functional term. In previous DFT reports [18,19], they mentioned that the GGA-PBE functional presents the optimum of calculation time, and the outputs of TMdoped MgH2 systems are in consistent with using the hybrid
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 1 e Unit cells of MgH2 in a; b and g structures and their super cells with TM-doped (Mg31TMH64). functional such as HSE06. The supercells of 32MgH2 in three structures were generated, and substituted by Ni, Pd or Pt dopants. For geometry optimizations, the optimum cutoff energy was set at 500 eV. The ultrasoft-pseudopotential was used as the scheme of pseudo-potential term in Kohn-Sham equations. The condition of k-point sampling grid size for calculations was set at 1 =kz0:025in each dimension. Forces on the optimized atomic positions in each step of optimization were controlled through the Hellmann-Feynman theorem [26]. The hydrostatic pressure, which contained equivalent forces on a supercell, was evaluated using the third order birch-murnaghan equation of state [27,28].
Results and discussion Phase transitions of TM-doped MgH2 To study the physical properties of Mg1-xTMxH2 in a; b and g structures, the supercells of 96 atoms were initially generated from the undoped conditions and optimized as shown in Fig. 1. A considerate TM atom, which consists of Ni, Pd or Pt, was substituted on a Mg site in a supercell, presenting the chemical formula Mg31 (TM) H64. To verify basic parameters from our optimizations presented in Table 1, we can see that
Table 1 e The calculated lattice vectors of pure and doped MgH2 in three structures at 0 GPa compared to the previous experiment [7] and DFT [8,19] reports. System a b (A) c Ref. a Mg32 H64
b Mg32 H64
g Mg32 H64
a Mg31 NiH64 a Mg31 PdH64 a Mg31 PtH64 b Mg31 NiH64 b Mg31 PdH64 b Mg31 PtH64 g Mg31 NiH64 g Mg31 PdH64 g Mg31 PtH64
4.5340 4.4853 4.5010 4.8393 4.7902 4.7934 4.5293 4.4860 4.5010 4.4871 4.5157 4.5232 4.5257 4.8204 4.7752 4.8336 4.8386 4.5138 4.4675 4.5232 4.5304
4.5340 4.4853 4.5010 4.8393 4.7902 4.7934 5.4735 5.4024 5.4197 5.4188 4.5157 4.5232 4.5257 4.8204 4.7752 4.8336 4.8386 5.4269 5.3974 5.4347 5.4339
3.0288 2.9993 3.0100 4.8393 4.7902 4.7934 4.9518 4.8985 4.9168 4.9066 3.0201 3.0342 3.0363 4.8204 4.7752 4.8336 4.8386 4.9622 4.8858 4.9878 4.9973
This work DFT [8] Exp [7] This work DFT [8] DFT [19] This work DFT [8] Exp [7] DFT [19] This work This work This work This work DFT [19] This work This work This work DFT [19] This work This work
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 2 e Phase transitions of undoped and TM-doped MgH2 under pressure up to 10 GPa.
all lattice parameters, for both doped and undoped MgH2, are in good agreement with the previous DFT [8,19] and experiment [7] reports. All percentage errors are within 1% comparisons to the experiment [7]. The optimized TM-doped structures started the initial conditions from the and phases of pure MgH2 [8], which are in space groups P42/mnm, Pa3 and Pbcn, respectively. Structural phase stability under pressure is determined from a minimum free energy structure at a given pressure. For total free energy comparisons through an enthalpy of system, the enthalpy difference (HeH1) is enthalpy in another phase (H) based on the ambient pressure -phase (H1). The enthalpy differences at 0, 5 and 10 GPa are presented in Fig. 2. The calculated phase transitions of pure MgH2 supported the previous DFT study [8] that the ambient pressure phase in changed to at 0.3 GPa, and then it transformed to the phase at 8.1 GPa. In the doped conditions, the phase has become the stable phase at 0 GPa because the decreasing of enthalpy is dominated in the and phases. The effect of atomic sizes was compared by the Ni, Pd or Pt replacements, and it is seen that the value of the
Table 2 e The H-vacancy enthalpies of Ni-, Pd- and Ptdoped 32MgH2 at 0, 5 and 10 GPa. System
Fig. 3 e Occupation enthalpy of TM-doped MgH2 under pressure.
a Mg31 NiH64 b Mg31 NiH64 g Mg31 NiH64 a Mg31 PdH64 b Mg31 PdH64 g Mg31 PdH64 a Mg31 PtH64 b Mg31 PtH64 g Mg31 PtH64
Hvac (eV) 0 GPa
5 GPa
10 GPa
3.74 3.50 4.15 3.82 3.21 4.25 3.77 3.27 4.70
3.66 3.43 3.96 3.70 3.11 4.02 3.64 3.14 4.59
3.58 3.39 3.76 3.60 3.02 3.87 3.53 3.03 4.55
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 4 e Labels of six neighbor atoms in the optimized structures of Ni-doped MgH2 at 0 GPa.
transition pressure of had shifted to the higher pressure depending on the heavier atomic size (Pt > Pd > Ni).
Occupation and H-vacancy enthalpies To analyze a heat of forming compound under pressure, the occupation enthalpy (Hocc) of an optimized system can be expressed as Hocc ¼ HMg31 H64 TM 32H32MgH2 HTM þ HMg
(1)
The Hocc is used to identify the preferential site for dopants in bulk compared to the undoped condition. The negative value of Hocc indicates an exothermic process, while an endothermic process requires the absorption energy or positive enthalpy change. In Fig. 3, the tendencies of occupation enthalpy under pressure are correlated with the phase transitions. The decreasing of slopes in three lines indicates that the high-pressure condition induced the substitution of Ni, Pd and Pt dopants into MgH2. The occupation of TM substitution in b and g structures is easier than in the ambient pressure phase, as indicated by the higher negative slopes. In addition, we also analyzed the formation enthalpy of hydrogen vacancy in the doped system. The ability of the occurrence of H-vacancy in a supercell can be determined in terms of the H-vacancy enthalpy (Hvac), expressed as
and three groups in the low-symmetry g-phase that are B1-2, B3-4 and B5-6. The minimum free energy with H removal in each compound is presented in Table 2, and then the diffusion paths and potential barriers of H-vacancy translation in space between nearby atoms Hi are investigated. It was assumed as the estimated direct paths. The calculated activation energy barriers of H-vacancy in pure and doped MgH2 systems in a; b and g phases are observed as shown in Table 3, and the result in undoped condition is compared to the previous DFT report [20]. The barrier is equal to 0.83 eV when the system is estimated as Mg54H107, which was previously reported in the range of 0.79e1.10 eV [20]. The italic text in Table 3 presents the minimum activation energy barriers in each condition, which were selected to present the pathway curves in Fig. 5.
Table 3 e The calculated activation energy barriers of Hvacancy in the undoped and TM-doped MgH2 systems. *The value is equal to 0.83 eV when the system is estimated as Mg54H107, which the barriers were reported in the range of 0.79e1.10 eV [20]. The italic text presents the minimum barriers in each condition. Impurity Diffusion Path
a phase b phase g phase undoped
1 Hvac ¼ HMg31 H63 TM HMg31 H64 TM þ HH2 2
(2)
The calculated results of H-vacancy enthalpy of TM-doped 32MgH2 at 0, 5 and 10 GPa are presented in Table 2, showing that Hvac decreases under high pressure. Therefore, it can be concluded that high pressure induces the occupation of H-vacancy in both pure and TM-doped MgH2 also.
Diffusion kinetics of H-vacancy around the TM atom Later, we also analyzed the diffusion kinetics of H-vacancy around the TM atom into the supercell lattice. H atoms around the TM atom are labeled in Fig. 4. The chemical bonding of TM-Hi (Bi) is used to compare the bond length between the TM atom and the ith H atom; i ¼ 1,2,3, …, 6. All Bi bonds in the high-symmetry cubic b-phase are equivalent, while it has two groups in the a-phase that are B1-4 and B5-6,
Energy Barriers (meV/atom)
Ni-doped
Pd-doped
Pt-doped
1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6
14.8* 18.4 18.4 30.8 18.4 13.8 16.4 16.4 17.2 16.5 16.2 24.9 24.0 24.6 24.2 21.9 29.2 29.2 26.3 39.5
10.8 28.1 10.8 28.1 28.1 12.1 19.7 12.1 19.6 19.6 16.4 30.2 16.4 30.2 30.2 23.9 33.1 24.3 33.6 32.9
16.6 23.3 26.9 34.3 13.5 23.3 12.0 11.0 21.1 12.9 35.3 13.2 12.3 35.8 13.3 40.6 12.3 9.8 39.4 12.5
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 5 e Comparisons of H-vacancy diffusion curves in three structures at 0 and 5 GPa.
In the results of the H-vacancy diffusion kinetics shown in Fig. 5, we found that the diffusion barriers at ambient pressure vary in the range of 8e24 meV. It is well known that all compounds commonly increase at high pressure due to the stimulated stress from external forces. The extreme external stress does not give the better solution. Therefore, the curves at 10 GPa are neglected in the presentation. In between three phases, the result in Fig. 5a shows that the b phase possesses the minimum barrier in pure MgH2 at 10.8 meV, while it is 13.5 meV in the g phase and 14.8 meV in the a phase. The highpressure phases in b and g reduced the activation barrier from the ambient phase (a) of pure MgH2. For the Ni-doped MgH2 presented in Fig. 5b, the activation barriers in pure MgH2 are induced by the Ni dopant, and the activation curve in the g phase at 0 GPa presents the minimum barrier. The initial and final points of translation curves in the TM-doped conditions have an energy difference because TM doping gave the lower symmetry than pure MgH2 did. However, we can see that the barriers in Pd and Pt are significantly decreased in the g phase, as shown in Fig. 5c and d.
Conclusion In summary, the structural properties and internal mechanisms of Mg31TMH64 have been presented. Phase transition,
formation enthalpy, H-vacancy enthalpy and activation barrier curves in a; b and g phases are analyzed. The g/b transition pressure in TM-doped MgH2 shifts to high pressure. The g-phase becomes the more stable phase at ambient pressure. High pressure induces the substitution of Ni, Pd and Pt dopants into MgH2 and the occupation of H-vacancy in both pure and TM-doped MgH2 also. The activation barriers in pure MgH2 are induced by the Ni dopant, and the activation curve in the g phase at 0 GPa presents the minimum barrier. The minimum barriers of TM-doped MgH2 are significantly dominated in the g phase.
Acknowledgements This work has been partially supported by Chiang Mai University, National Research Council of Thailand, the Center of Excellence in Materials Science and Technology, the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education. Computing facilities have been supported by Super SCI-IV research grant, Faculty of Science and Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University. P. P. would like to acknowledge the financial supports and facilities from Huachiew Chalermprakiet University and Thailand Research Fund (TRF) MRG6080231.
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement Prayoonsak Pluengphon a,*, Thiti Bovornratanaraks b,c, Prutthipong Tsuppayakorn-aek b,c, Udomsilp Pinsook b,c, Burapat Inceesungvorn d,** a
Division of Physical Science, Faculty of Science and Technology, Huachiew Chalermprakiet University, Samutprakarn, 10540, Thailand b Extreme Conditions Physics Research Laboratory (ECPRL) and Physics of Energy Materials Research Unit (PEMRU), Department of Physics, Faculty of Science, Chulalongkorn University, 10330, Bangkok, Thailand c Thailand Center of Excellence in Physics, Commission on Higher Education, 328 Si-Ayuttaya Road, Bangkok, 10400, Thailand d Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Center of Excellence in Materials Science and Technology, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand
highlights
graphical abstract
The g-phase has possessed the minimum free energy in the TMdoped systems. The
occupation
of
TM
sub-
stitutions in the b and g phases is easier than in the a phase. High-pressure induces the occupation
of
H-vacancy
into
the
compounds. The activation energy curves of Ni-, Pd- and Pt-doped MgH2 under pressure are presented. The minimum activation energy barriers are significantly dominated in the g phase.
article info
abstract
Article history:
The improvement of the hydrogen storage mechanism in TM-doped MgH2 by structural
Received 2 May 2019
high-pressure effects has been found using ab initio calculation. Phase transition, formation
Received in revised form
enthalpy and H-vacancy mechanism of a-, b-, and g-MgH2 with 3.125% of Ni, Pd and Pd
28 May 2019
dopants are analyzed under the pressure conditions of 0, 5 and 10 GPa. It is found that the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Pluengphon), [email protected] (B. Inceesungvorn). https://doi.org/10.1016/j.ijhydene.2019.06.066 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
2 Accepted 11 June 2019 Available online xxx
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enthalpy of b- and g-phases based on the a-phase decreases in TM-doped systems, especially for the heavier atomic size of dopants (Pt > Pd > Ni). As a result, the g-phase has become structural stable at ambient pressure. The occupation enthalpy of TM substitutions
Keywords:
in b and g phases is easier than that in the a phase, which indicates ability of mixing
Doped-MgH2
impurities. High pressure induces the occupation of H-vacancy in all compounds. The
H-vacancy
activation energy curves of MgH2 with Ni, Pd and Pt dopants are also analyzed, and the
Kinetics diffusion
minimal barriers are significantly dominated in the g phase.
Hydrogen storage
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High pressure
Introduction Hydrogen energy is one of the most vivid solutions for finding a future fuel source, especially in vehicle technology, due to its nontoxic gas emission and unlimited source on the earth. Using a hydrogen energy carrier by directly containing gas in a tank is at a too low efficiency level for burning fuel for a long time in mobile vehicles [1e4]. Therefore, hydrogen storage methods have been widely studied for enhancing the efficiency of the energy carrier and internal process [1,3]. Solid state compounds which are used in the hydrogen storage method transfer hydrogen gas through chemical methods. A hydrogen storage process with a metal hydride group [5,6] has been initially presented as a new way to store hydrogen energy, and it has the special capability to reversibly store and release hydrogen [3,5]. Magnesium hydride (MgH2) is one of the candidate practical materials used in on-board hydrogen storage in vehicles because of its low cost and high gravimetric and volumetric capacities of H2 (7.7 wt% and 109 gH2) [5]. However, pure MgH2 gives slow reaction kinetics and a high temperature of absorption/desorption processes. In the past, crystal structures and phase transitions of MgH2 under high pressure were intensively studied in both theory and experiment investigations [7e9]. By investigation of X-ray powder and neutron diffractions [7], it was found that the ambient phase aMgH2 transforms to an g-MgH2 orthorhombic phase at the pressure 2 GPa. The theoretical study based on density functional theory (DFT) [8] presented the existence of other highpressure phases that are a; b and g structures. Later, the DFT result was verified by a high-pressure synchrotron X-ray diffraction technique [9]. The reaction enthalpy of pure MgH2 at 0 K was reported as 64.7 kJ/mol H2 [10]. A large value of heat in the adsorption-desorption process is a main problem for improvement. There is recent progress in tuning of diffusion kinetics or thermodynamic properties by additive compositions [11,12]. Improvement of H2 adsorption-desorption in MgH2 has been widely introduced by adding a transition metal (TM) as an additive or dopant [13e22]. The desorption temperature and dehydrogenation enthalpy of the MgeNb2O5 mixtures were lower than that of pure MgH2 [13]. The free energies with Fe catalysts indicated the lower desorption temperature [14]. Liang et al. [15] studied ball-milled MgH2-TM nanocomposites with Ti, V, Mn, Fe and Ni. They showed that the activation energy of desorption for MgH2 was significantly decreased by milling with TM. Hanada et al. [16] catalyzed a
MgH2 composite by milling with a TM-nanoparticle of Fe, Co, Ni, and Cu. They found that Ni doping gave the minimum activation energy for hydrogen desorption. Kodera et al. [17] studied the Mg2NiH4 compound using the mechanical grinding process and found that it decreases absorption pressures and enhances the reaction kinetics. Later, the DFT investigations of TM-doped a-, g-, and b-MgH2 were analyzed [18,19]. The Ni dopant significantly decreases the lattice constants. The TM dopants influence the stability of both b- and gMgH2, and they can decrease the hydrogen desorption from band g-MgH2 at much lower temperatures than from pure MgH2 [19]. German and Gabauer [20] presented the improvement of hydrogen vacancy diffusion kinetics in a-MgH2 by Nb and Zr dopants, which was determined from activation energy curves. Moreover, the PdeNi bimetallic dopants were synthesized by an experiment with mesoporous carbon [21] that presented enhanced dehydrogenation performance in MgH2. Doping effect on the layers of MgH2 (110) surface was observed [22] and found that the impurity at the first and second layers presented the lowering of H2 desorption. In this study, we found that the diffusion kinetics of Hvacancy in TM-doped MgH2 at high-pressure phases remains incomplete. Therefore, this report is focused on the hydrogen diffusion kinetics mechanisms in the three first phases (a; b and g) of MgH2 with Ni, Pd and Pt dopants. Structural phase transition, formation enthalpy, occurrence of H-vacancy enthalpy and activation energy curves are intensively investigated using ab initio study. Our calculation results indicate that high-pressure structures induce the better solutions for improving of the mechanism of hydrogen desorption and the energy barrier of H-vacancy diffusion kinetics.
Calculation details In this research, an ab initio calculation based on DFT was performed to solve Kohn-Sham equations [23] as implement in Cambridge Serial Total Energy Package (CASTEP) [24]. The minimum free energy, effective potential and electron density at a ground state condition were evaluated by self-consistent field method [23]. The generalized-gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) [25] was selected as the exchange-correlation functional term. In previous DFT reports [18,19], they mentioned that the GGA-PBE functional presents the optimum of calculation time, and the outputs of TMdoped MgH2 systems are in consistent with using the hybrid
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 1 e Unit cells of MgH2 in a; b and g structures and their super cells with TM-doped (Mg31TMH64). functional such as HSE06. The supercells of 32MgH2 in three structures were generated, and substituted by Ni, Pd or Pt dopants. For geometry optimizations, the optimum cutoff energy was set at 500 eV. The ultrasoft-pseudopotential was used as the scheme of pseudo-potential term in Kohn-Sham equations. The condition of k-point sampling grid size for calculations was set at 1 =kz0:025in each dimension. Forces on the optimized atomic positions in each step of optimization were controlled through the Hellmann-Feynman theorem [26]. The hydrostatic pressure, which contained equivalent forces on a supercell, was evaluated using the third order birch-murnaghan equation of state [27,28].
Results and discussion Phase transitions of TM-doped MgH2 To study the physical properties of Mg1-xTMxH2 in a; b and g structures, the supercells of 96 atoms were initially generated from the undoped conditions and optimized as shown in Fig. 1. A considerate TM atom, which consists of Ni, Pd or Pt, was substituted on a Mg site in a supercell, presenting the chemical formula Mg31 (TM) H64. To verify basic parameters from our optimizations presented in Table 1, we can see that
Table 1 e The calculated lattice vectors of pure and doped MgH2 in three structures at 0 GPa compared to the previous experiment [7] and DFT [8,19] reports. System a b (A) c Ref. a Mg32 H64
b Mg32 H64
g Mg32 H64
a Mg31 NiH64 a Mg31 PdH64 a Mg31 PtH64 b Mg31 NiH64 b Mg31 PdH64 b Mg31 PtH64 g Mg31 NiH64 g Mg31 PdH64 g Mg31 PtH64
4.5340 4.4853 4.5010 4.8393 4.7902 4.7934 4.5293 4.4860 4.5010 4.4871 4.5157 4.5232 4.5257 4.8204 4.7752 4.8336 4.8386 4.5138 4.4675 4.5232 4.5304
4.5340 4.4853 4.5010 4.8393 4.7902 4.7934 5.4735 5.4024 5.4197 5.4188 4.5157 4.5232 4.5257 4.8204 4.7752 4.8336 4.8386 5.4269 5.3974 5.4347 5.4339
3.0288 2.9993 3.0100 4.8393 4.7902 4.7934 4.9518 4.8985 4.9168 4.9066 3.0201 3.0342 3.0363 4.8204 4.7752 4.8336 4.8386 4.9622 4.8858 4.9878 4.9973
This work DFT [8] Exp [7] This work DFT [8] DFT [19] This work DFT [8] Exp [7] DFT [19] This work This work This work This work DFT [19] This work This work This work DFT [19] This work This work
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 2 e Phase transitions of undoped and TM-doped MgH2 under pressure up to 10 GPa.
all lattice parameters, for both doped and undoped MgH2, are in good agreement with the previous DFT [8,19] and experiment [7] reports. All percentage errors are within 1% comparisons to the experiment [7]. The optimized TM-doped structures started the initial conditions from the and phases of pure MgH2 [8], which are in space groups P42/mnm, Pa3 and Pbcn, respectively. Structural phase stability under pressure is determined from a minimum free energy structure at a given pressure. For total free energy comparisons through an enthalpy of system, the enthalpy difference (HeH1) is enthalpy in another phase (H) based on the ambient pressure -phase (H1). The enthalpy differences at 0, 5 and 10 GPa are presented in Fig. 2. The calculated phase transitions of pure MgH2 supported the previous DFT study [8] that the ambient pressure phase in changed to at 0.3 GPa, and then it transformed to the phase at 8.1 GPa. In the doped conditions, the phase has become the stable phase at 0 GPa because the decreasing of enthalpy is dominated in the and phases. The effect of atomic sizes was compared by the Ni, Pd or Pt replacements, and it is seen that the value of the
Table 2 e The H-vacancy enthalpies of Ni-, Pd- and Ptdoped 32MgH2 at 0, 5 and 10 GPa. System
Fig. 3 e Occupation enthalpy of TM-doped MgH2 under pressure.
a Mg31 NiH64 b Mg31 NiH64 g Mg31 NiH64 a Mg31 PdH64 b Mg31 PdH64 g Mg31 PdH64 a Mg31 PtH64 b Mg31 PtH64 g Mg31 PtH64
Hvac (eV) 0 GPa
5 GPa
10 GPa
3.74 3.50 4.15 3.82 3.21 4.25 3.77 3.27 4.70
3.66 3.43 3.96 3.70 3.11 4.02 3.64 3.14 4.59
3.58 3.39 3.76 3.60 3.02 3.87 3.53 3.03 4.55
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 4 e Labels of six neighbor atoms in the optimized structures of Ni-doped MgH2 at 0 GPa.
transition pressure of had shifted to the higher pressure depending on the heavier atomic size (Pt > Pd > Ni).
Occupation and H-vacancy enthalpies To analyze a heat of forming compound under pressure, the occupation enthalpy (Hocc) of an optimized system can be expressed as Hocc ¼ HMg31 H64 TM 32H32MgH2 HTM þ HMg
(1)
The Hocc is used to identify the preferential site for dopants in bulk compared to the undoped condition. The negative value of Hocc indicates an exothermic process, while an endothermic process requires the absorption energy or positive enthalpy change. In Fig. 3, the tendencies of occupation enthalpy under pressure are correlated with the phase transitions. The decreasing of slopes in three lines indicates that the high-pressure condition induced the substitution of Ni, Pd and Pt dopants into MgH2. The occupation of TM substitution in b and g structures is easier than in the ambient pressure phase, as indicated by the higher negative slopes. In addition, we also analyzed the formation enthalpy of hydrogen vacancy in the doped system. The ability of the occurrence of H-vacancy in a supercell can be determined in terms of the H-vacancy enthalpy (Hvac), expressed as
and three groups in the low-symmetry g-phase that are B1-2, B3-4 and B5-6. The minimum free energy with H removal in each compound is presented in Table 2, and then the diffusion paths and potential barriers of H-vacancy translation in space between nearby atoms Hi are investigated. It was assumed as the estimated direct paths. The calculated activation energy barriers of H-vacancy in pure and doped MgH2 systems in a; b and g phases are observed as shown in Table 3, and the result in undoped condition is compared to the previous DFT report [20]. The barrier is equal to 0.83 eV when the system is estimated as Mg54H107, which was previously reported in the range of 0.79e1.10 eV [20]. The italic text in Table 3 presents the minimum activation energy barriers in each condition, which were selected to present the pathway curves in Fig. 5.
Table 3 e The calculated activation energy barriers of Hvacancy in the undoped and TM-doped MgH2 systems. *The value is equal to 0.83 eV when the system is estimated as Mg54H107, which the barriers were reported in the range of 0.79e1.10 eV [20]. The italic text presents the minimum barriers in each condition. Impurity Diffusion Path
a phase b phase g phase undoped
1 Hvac ¼ HMg31 H63 TM HMg31 H64 TM þ HH2 2
(2)
The calculated results of H-vacancy enthalpy of TM-doped 32MgH2 at 0, 5 and 10 GPa are presented in Table 2, showing that Hvac decreases under high pressure. Therefore, it can be concluded that high pressure induces the occupation of H-vacancy in both pure and TM-doped MgH2 also.
Diffusion kinetics of H-vacancy around the TM atom Later, we also analyzed the diffusion kinetics of H-vacancy around the TM atom into the supercell lattice. H atoms around the TM atom are labeled in Fig. 4. The chemical bonding of TM-Hi (Bi) is used to compare the bond length between the TM atom and the ith H atom; i ¼ 1,2,3, …, 6. All Bi bonds in the high-symmetry cubic b-phase are equivalent, while it has two groups in the a-phase that are B1-4 and B5-6,
Energy Barriers (meV/atom)
Ni-doped
Pd-doped
Pt-doped
1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6 1/4 1/5 1/6 1/2 3/6
14.8* 18.4 18.4 30.8 18.4 13.8 16.4 16.4 17.2 16.5 16.2 24.9 24.0 24.6 24.2 21.9 29.2 29.2 26.3 39.5
10.8 28.1 10.8 28.1 28.1 12.1 19.7 12.1 19.6 19.6 16.4 30.2 16.4 30.2 30.2 23.9 33.1 24.3 33.6 32.9
16.6 23.3 26.9 34.3 13.5 23.3 12.0 11.0 21.1 12.9 35.3 13.2 12.3 35.8 13.3 40.6 12.3 9.8 39.4 12.5
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Fig. 5 e Comparisons of H-vacancy diffusion curves in three structures at 0 and 5 GPa.
In the results of the H-vacancy diffusion kinetics shown in Fig. 5, we found that the diffusion barriers at ambient pressure vary in the range of 8e24 meV. It is well known that all compounds commonly increase at high pressure due to the stimulated stress from external forces. The extreme external stress does not give the better solution. Therefore, the curves at 10 GPa are neglected in the presentation. In between three phases, the result in Fig. 5a shows that the b phase possesses the minimum barrier in pure MgH2 at 10.8 meV, while it is 13.5 meV in the g phase and 14.8 meV in the a phase. The highpressure phases in b and g reduced the activation barrier from the ambient phase (a) of pure MgH2. For the Ni-doped MgH2 presented in Fig. 5b, the activation barriers in pure MgH2 are induced by the Ni dopant, and the activation curve in the g phase at 0 GPa presents the minimum barrier. The initial and final points of translation curves in the TM-doped conditions have an energy difference because TM doping gave the lower symmetry than pure MgH2 did. However, we can see that the barriers in Pd and Pt are significantly decreased in the g phase, as shown in Fig. 5c and d.
Conclusion In summary, the structural properties and internal mechanisms of Mg31TMH64 have been presented. Phase transition,
formation enthalpy, H-vacancy enthalpy and activation barrier curves in a; b and g phases are analyzed. The g/b transition pressure in TM-doped MgH2 shifts to high pressure. The g-phase becomes the more stable phase at ambient pressure. High pressure induces the substitution of Ni, Pd and Pt dopants into MgH2 and the occupation of H-vacancy in both pure and TM-doped MgH2 also. The activation barriers in pure MgH2 are induced by the Ni dopant, and the activation curve in the g phase at 0 GPa presents the minimum barrier. The minimum barriers of TM-doped MgH2 are significantly dominated in the g phase.
Acknowledgements This work has been partially supported by Chiang Mai University, National Research Council of Thailand, the Center of Excellence in Materials Science and Technology, the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education. Computing facilities have been supported by Super SCI-IV research grant, Faculty of Science and Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University. P. P. would like to acknowledge the financial supports and facilities from Huachiew Chalermprakiet University and Thailand Research Fund (TRF) MRG6080231.
Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066
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Please cite this article as: Pluengphon P et al., High-pressure phases induce H-vacancy diffusion kinetics in TM-doped MgH2: Ab initio study for hydrogen storage improvement, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.066