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Crystal Structures, Phase Stability, and Decomposition Reactions in the Quaternary Mg−B−N−H Hydrogen Storage System Yongsheng Zhang,*,† David Farrell,‡ Jun Yang,‡ Andrea Sudik,‡ and C. Wolverton† †

Department of Materials Science & Engineering, Northwestern University, Evanston, Illinois 60208, United States Ford Motor Company Research and Advanced Engineering, 2101 Village Rd, RIC/MD1170, Dearborn, Michigan 48121, United States

‡

ABSTRACT: Using the combination of DFT-based computational approaches and experimental measurements, we have studied the crystal structure, phase stability, and decomposition products of mixed Mg(NH2)2/Mg(BH4)2 materials. We find the following: (i) DFT crystal structure prediction calculations (0 K) show the existence of a mixed Mg(NH2)2/Mg(BH4)2 phase, which is thermodynamically stable relative to its separated phases [Mg(NH2)2 and Mg(BH4)2]. (ii) The DFT calculated phonon density of states of Mg(NH2)(BH4) is in good agreement with the peak positions from experimental PAS IR measurements (at the room temperature) of a ball-milled Mg(NH2)2/Mg(BH4)2 mixture, suggesting the mixture is not merely a physical mixture of the individual compounds. (iii) The experimentally measured dehydrogenation temperature of the mixed Mg(NH2)2/Mg(BH4)2 phase is lower than that of Mg(NH2)2 or Mg(BH4)2, which further confirms that it is not a simply physical mixture of Mg(NH2)2 and Mg(BH4)2. The observed amount of H2 release is 3.4 wt % at 250° and 8.3 wt % above 280°. (iv) From a combination of DFT, the grand-canonical linear programming (GCLP) method calculations, and PAS IR measurements of dehydrogenated samples, we identify the existence of the B−H bonds and linear N−B−N units in the decomposition of Mg(NH2)2/Mg(BH4)2. (v) Experimental desorption measurements reveal that the Mg(NH2)2/Mg(BH4)2 mixed phase is irreversible, consistent with DFT calculated enthalpies in the range of −18 to +16 kJ/(mol H2), too low for nearambient reversible storage.

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INTRODUCTION Metal borohydrides1−10 and metal amides11−14 [M(BH4)n and M(NH2)n, where M is an alkali or alkaline earth metal] are receiving considerable attention as hydrogen storage materials due to their high gravimetric capacities of hydrogen. Unfortunately, these compounds suffer from thermodynamic and/or kinetic problems during dehydrogenation/rehyrdogenation, leading to significantly higher H2 release temperatures than what is suitable for automotive fuel cell applications (approximately 80 °C). Amides and borohydrides also have an observed tendency to release undesirable byproducts (e.g., ammonia or diborane). Recently, several mixed metal (Li−Mg−N−H,15−17 Li−Ca− N−H,18,19 Li−Zn−B−H,20−22 Li−Al−N−H,23,24 Li−Mg−Al− H25) and mixed anion (Li−B−N−H,26−31 Na−B−N−H,32,33 Ca−B−N−H38) hydrogen storage materials have been studied both experimentally and computationally. Very recently, the crystal structure of Mg(NH2)(BH4) has been identified as having the I41 (80) space group.34 These quaternary systems often show better thermodynamic and kinetic properties (e.g., lower desorption temperatures) during dehydrogenation than their corresponding separated phases. For example, LiNH2 releases NH3 during decomposition35,36 (2LiNH2 → Li2NH + NH3), and the H2 release temperature for LiBH4 is too high,37 >400 °C. Pinkerton et al.26 synthesized a quaternary phase [Li3(NH2)2(BH4) or Li3BN2H8] by ball milling LiNH2 © 2014 American Chemical Society

and LiBH4 powder in a 2:1 molar ratio, and found that Li3BN2H8 releases ≥10 wt % H2 above ∼250 °C. Chater et al.32 and Somer et al.33 synthesized another mixed complex hydride, Na2NH2BH4 (Na2NBH6), which releases ∼8 wt % H2 above ∼300 °C. These experimental studies suggest that mixed complex hydrides [NH2]/[BH4] could be potential candidates for solid hydrogen storage materials with improved properties. In this work, we focus on a quaternary hydrogen storage material, in the Mg−B−N−H system, a mixture of Mg(BH4)2 and Mg(NH2)2. We combine both experimental measurements and theoretical calculations to study the phase stability and crystal structures of the Mg(NH2)2 + Mg(BH4)2 mixture and dehydrogenation products and to characterize the reversibility of the mixture. Density-functional theory (DFT) has been used to accurately predict thermodynamic properties of many H2 storage materials,8−10,38−62 such as determining low-energy crystal structures, new decomposition products, and hydrogen uptake/release temperature−pressure conditions. Here, we use DFT to study the structural and thermodynamic properties of the Mg−B−N−H quaternary system. In order to carry out DFT calculations of possible Mg−B−N−H phases, the candidate crystal structures must first be identified. However, Received: January 10, 2014 Revised: April 11, 2014 Published: May 6, 2014 11193

dx.doi.org/10.1021/jp500318e | J. Phys. Chem. C 2014, 118, 11193−11202

The Journal of Physical Chemistry C

Article

Majzoub,67 was based on the recognition that atomic positions in some complex hydrides (e.g., the Na and Al positions in NaAlH4 and Na3AlH6) coincide with electrostatic point charge ground states.68 In the PEGS method, we describe the mixed system as a collection of charged cations and rigid complex anion groups (e.g., [NH2]− and [BH4]−) interacting via electrostatic potential and soft sphere repulsion:

to date, only the crystal structure of mixed Mg(NH2)2/ Mg(BH4)2 at 1:1 mixing ratio is experimentally known.34 A recently developed Monte Carlo based prototype electrostatic ground state (PEGS) method67 has been used to successfully predict low-energy crystal structures of complex hydrides containing a wide range of anion groups such as [AlH4]− and [AlH6]3− (ref 67), [BH4]− (refs 21 and 39−41), [BnHm]x− (refs 8−10), [NH2BH3]− (ref 42), the hypophosphite anion [BH3PH2BH3]− (ref 44), an amorphous AlB4H11 (ref 43), and [NH2]− + [BH4]− (ref 38). In ref 38, Aidhy et al. have used PEGS to predict compounds in Ca−B−N−H. Following these previous studies, we use PEGS to predict the crystal structure of possible mixed Mg(NH2)2/Mg(BH4)2 phases. In particular, PEGS + DFT calculations predict the stability of a Mg(NH2)(BH4) structure [relative to its separated phases Mg(NH2)2 + Mg(BH4)2]. The DFT energies of the theoretically predicted structure and experimentally determined structure are degenerate with 13 meV/f.u. The DFT calculated phonon density of states of Mg(NH2)(BH4) shows good agreement with the peak positions from experimental IR measurements. The experimentally measured dehydrogenation temperature of the mixed Mg(NH2)2/Mg(BH4)2 phase is lower than that of Mg(NH2)2 and Mg(BH4)2, which confirms that the mixed Mg(NH2)2/ Mg(BH4)2 phase is not a simple physical mixture of Mg(NH2)2 and Mg(BH4)2. Finally, the low-energy decomposition pathways and reversibilty of the mixed compound are investigated.

PEGS Etot

⎧ ⎪∑ ⎪ ⎪ i>j =⎨ ⎪ ⎪∑ ⎪ i>j ⎩

Q iQ j dij

+

∑ i>j

Q iQ j

1 dij12

dij < (R i + R j)

dij ≥ (R i + R j)

dij

(1)

where each atom i, including those comprising the complex anions, is represented by a radius (Ri) and a charge (Qi), and dij is the separation distance between atoms i and j. The Coulomb interactions, represented by the first term in eq 1, are calculated for all pairs of atoms regardless of distance, while the softsphere interactions (the second term in eq 1) are only nonzero when atomic spheres overlap. We take the ionic radius of the Mg cation from standard sources69 (RMg = 0.65 Å), and its ionic charge was given the nominal value of +2e. For the anion groups, the charges distributed in each anion group ([BH4]− and [NH2]−) were computed by the GAMESS cluster code.70 The radii of B and H in the [BH4]− unit were taken from previous Mg(BH4)2 PEGS + DFT crystal structure predictions.39 The N radius in the [NH2]− unit was calculated from the difference between the interatomic distance of Mg and N [d(Mg−N)] in Mg(NH2)2 and RMg, and the H radius was taken from our previously published work.42 We have summarized the anion group parameters in Table 1. After determining the

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THEORETICAL METHODS Density-Functional Theory. We performed DFT calculations using the Vienna Ab Initio Simulation Package (VASP) code with the projector-augmented wave (PAW) scheme64 and the generalized gradient approximation (GGA) of Perdew and Wang65 for the electronic exchange-correlation functional. The energy cutoff for the plane wave expansion of electronic wave functions was set to 875 eV. We treated 3s2 and 2s22p1 as valence electrons in Mg and B, respectively. The Brillouin zones were sampled by Monkhorst−Pack66 k-point meshes chosen to give a roughly constant density of k-points (30/Å−3) for all compounds. Tests showed that our choice of k-points yields energies that are converged to within 0.01 eV/(formula unit). Atomic positions and the unit cell were both relaxed until all the forces and components of the stress tensor were below 0.01 eV/Å and 0.2 kbar, respectively. Phonons were calculated using the supercell force constant method (as implemented in the program described in refs 55 and 59), and the vibrational entropies and enthalpies were obtained by directly summing over the calculated phonon frequencies. We performed phonon calculations on various compounds with various symmetries and unit cell sizes. In each case, the supercell was constructed to make sure each lattice direction is larger than 8 Å. Crystal Structure Prediction. In this work, we wish to predict crystal structures of possible mixed Mg(NH2)2/ Mg(BH4)2 phases. DFT calculations are typically quite accurate for complex hydride systems: for a series of metal complex hydrides, the hydrogen release enthalpies from DFT (GGA) are typically within ∼10 kJ/(mol H2) of experimental values,59,60,62 and the accuracy can be improved by applying a simple correction to the energy of the H2 molecule.63 However, prediction of unknown crystal structures directly from DFT is difficult due to the large configuration space which must be explored. Hence, we turned to the prototype electrostatic ground state (PEGS) search method as implemented within the PEGS code.67 The PEGS method, developed by Ozolins and

Table 1. Parameters of Cation Atom (Mg) and Anion Groups ([BH4]− and [NH2]−), Radii (R) and Charges (Q), Used in PEGS Simulationsa Q (e) R (Å)

Mg

B

H(B)

N

H(N)

+2 0.65

−0.22 0.93

−0.195 1.30

−1.09 1.50

0.045 1.40

H(B) and H(N) indicate H atoms in the [BH4]− and [NH2]− groups, respectively.

a

PEGS input parameters, the computationally inexpensive electrostatic potential (eq 1) was used in Monte Carlo simulated annealing runs (MC) to find candidate ground state crystal structures and more accurate DFT calculations were carried out (including a full relaxation) for each of the candidates of PEGS output structures. We selected the structure with the lowest DFT energy and used this energy in further stability studies described below (more details in refs 9, 10, 42, 43, and 67). Reaction Pathway Prediction. To determine the lowest energy reactions and products in the decomposition of Mg(NH2)2/Mg(BH4)2, we used the grand-canonical linear programming (GCLP) method53 with a pool of 25 candidate decomposition products taken from the following systems: Mg−B−N−H, Mg−B−H, Mg−N−H, Mg−B−N, B−N−H, Mg−H, Mg−N, Mg−B, BN, B, Mg, H2(gas) [taken from the Inorganic Crystal Structure Database (ICSD) and also, in some cases, from previous theoretical predictions]. We calculated both the relaxed DFT total energy and the phonon vibrational 11194



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through the sample (100 sccm flow rate, atmospheric pressure). The sample was subsequently heated at a rate of 5 °C/min from room temperature up to 550 °C. Data collection was performed using a PC with the MKS ProcessEye Professional software at a rate of 6 scans/min. Hydrogen Capacity and Reversibility: WDD. Waterdisplacement desorption (WDD) measurements were used to determine the gravimetric density of gas desorbed. Additionally, these experiments allowed for the evaluation of reversibility, by performing multiple discharge/recharge cycles with a single sample. The equipment used consisted of a tubular reactor made of stainless steel with an attached valve, into which the sample was loaded in the glovebox. This reactor was then removed from the glovebox and attached to a T-junction, with one leg leading to a UHP hydrogen bottle (for recharging) and the other leading to a length of plastic tubing which was purged and placed into the base of the water-displacement portion of the apparatus.71 The water-displacement portion consisted of a buret (500 mL, 10 mL graduations) that had been filled with water and then partially submerged in a large open beaker, creating a water column at approximately atmospheric pressure, that can be displaced by gas exiting the reactor (via the plastic tube inserted into the buret). The gravimetric density was calculated from the observed volume change in the water column, the density of hydrogen (assuming STP), and the initial sample mass. In order to control the heating of the sample, a heating unit was fitted onto the reactor and wrapped in a fiberglass insulating blanket. The heating unit consisted of two aluminum blocks shaped to fit the reactor, with a heater and thermocouple inserted close to the interface of the reactor and aluminum blocks. The heating rate and profile was controlled by a Watlow Winona temperature controller, and will be described in the relevant section of this report.

thermodynamic contributions of each compound. Assuming all solid phases are in equilibrium with a hydrogen gas reservoir at a chemical potential of μH2(T, p), we minimize the Gibbs free energy of the grand-canonical ensemble to find the thermodynamically preferred reaction pathways. The grand potential is defined as Ω(T , p) =

∑ xiFi(T ) − i

μ H ( T , p) 2

2

∑ xiniH i

(2)

subject to the mass-conservation constraints for non-hydrogen species: fs = ∑i xinsi = const. Fi(T) is the free energy of compound i, nHi is the number of hydrogen atoms in one formula unit of compound i, xi are the molar fractions of compound at a given composition, temperature, and pressure (p = 1 bar), nsi is the number of atoms of type s in one formula unit of compound i, and fs represents given molar ratios of nonhydrogen species. Following standard conventions, ∑s≠H fs = 1.

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EXPERIMENTAL METHODS Sample Preparation. The Mg(NH2)2/Mg(BH4)2 mixture was synthesized by milling in a Spex ball mill for 5 h with a 10:1 ball-to-sample mass ratio (sample mass: 2 g). Mg(BH4)2 was purchased (95%, Sigma-Aldrich) and Mg(NH2)2 was synthesized in-house by milling as-purchased MgH2 (95%, Gelest) in 1 bar of NH3 using a modified milling vial, followed by annealing at 300 °C for 10 h in 8 bar of NH3. All sample handling was performed in an MBraun Labmaster 130 glovebox with an argon atmosphere, maintained at less than 1 ppm of O2 and H2O levels. Infrared Spectroscopy. In order to characterize the vibrational spectrum and local environment for the samples, two forms of Fourier-transform infrared spectroscopy (IR) were used: variable temperature diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS) and photoacoustic spectroscopy (PAS). The PAS instrument was a Mattson Instruments Cygnus 100 spectrometer, with a watercooled source and ancillary 75 Hz high-pass filter. We used an MTEC 200 PAS cell with a KBr window. The interferometer was tuned such that all of the light passed by the source was transferred through the 50% instrument iris aperture and onto the sample. The enclosure was purged with nitrogen, and the sample cell was purged with UHP helium. A glovebag attached to the front access panel of the enclosure was used to limit the exposure of the samples to air. A carbon black background was used and subtracted from the sample spectra based on 32 sample and 64 background scans for each spectrum. The interferometer mirror velocity was 0.08 cm/s, and all data analysis was performed in the Mattson WinFirst software. All PAS data were collected at room temperature. Desorbed Gas Purity: TPDMS. Temperature-programmed desorption mass spectrometry (TPDMS) was use to establish the purity of the hydrogen desorbed from the identified reactions as a function of temperature. The apparatus was constructed from in-house components, based on an MKS PPT electron−ionization quadrupole mass spectrometer equipped with a heated capillary inlet (80 °C) and a manifold heater (80 °C), a Lindberg tube furnace with programmable temperature controller, and a Brooks 5850 E-series mass flow controller. For each experiment, the sample (approximately 20 mg) was loaded into a quartz tube, held in place by quartz wool plugs, and sealed with two septa end-caps. The sample holder was placed into the furnace, and the UHP argon carrier gas was flowed

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RESULTS AND DISCUSSION PEGS + DFT Predicted Low-Energy Mg(NH2)2/Mg(BH4)2 Crystal Structures. PEGS is designed to identify lowenergy crystal structures for a known stoichiometry. However, in the mixed Mg(NH2)2/Mg(BH4)2 system, the correct stoichiometry of possible stable compound remains unknown. To search for possible mixed Mg(NH2)2/Mg(BH4)2 quaternary phases, we used PEGS + DFT to predict the low-energy structures of mixing Mg(NH2)2/Mg(BH4)2 at different ratios: Mg 2 (NH 2 ) 3 (BH 4 ), Mg 3 (NH 2 ) 4 (BH 4 ) 2 , Mg(NH 2 )(BH 4 ), Mg3(NH2)2(BH4)4, and Mg2(NH2)(BH4)3. To check the stability of the theoretically predicted phases, we calculated the mixing energy using the following: mix ΔEMg(NH 2)2x (BH4)2(1 − x)

= EMg(NH 2)2x (BH4)2(1−x) − xEMg(NH 2)2 − (1 − x)EMg(BH4)2 (3)

where x is the composition of Mg(NH2)2 and Ephase is the total energy of each phase (per Mg cation). Positive and negative mixing energies (ΔEmix in eq 3) indicate unstable and stable Mg(NH2)2/Mg(BH4)2 mixed phases, respectively, relative to its constituent phases [Mg(NH2)2 and Mg(BH4)2]. Using eq 3, the mixing energies of our theoretically predicted Mg2(NH2)3(BH4), Mg3(NH2)4(BH4)2, Mg3(NH2)2(BH4)4, and Mg2(NH2)(BH4)3 phases are positive (Figure 2), which indicates that these phases are thermodynamically unstable and will phase separate into Mg(NH2)2 and Mg(BH4)2. However, 11195



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indicates a prediction that a mixed quaternary phase should form thermodynamically during the mixing of Mg(NH2)2 and Mg(BH4)2. From a very recently published paper,34 Noritake et al. determined the Mg(NH2)(BH4) crystal structure experimentally (I41, 8 formula units, a = b = 5.8141 Å, c = 20.4504 Å). Since the experimental structure has 8 f.u., whereas we run PEGS + DFT only for 1−4 f.u., we knew in advance that the computational method cannot predict the observed structure. The PEGS + DFT theoretically predicted Mg(NH2)(BH4) crystal structure (Figure 1 and Table 2) has an Ima2 space group, which is different from the experimentally determined structure. Nevertheless, our PEGS structure captures the common local geometries of the experimental structure (Table 3): the distances of Mg−N and Mg−B, each Mg surrounded by 2N + 2B and layered Mg−NH2−BH4 structure. More important, the DFT energies of the two structures are degenerate within 1 kJ/(mol f.u.), which demonstrates the accuracy of the PEGS predictions. The excellent approximation to the true ground state energy is sufficient to study thermodynamic properties, such as hydrogen release enthalpy or reaction pathways. In ref 38, Aidhy et al. have used PEGS + DFT to predict a thermodynamically stable phase [Ca(NH2)(BH4) with a P-1 space group] in Ca(NH2)2/Ca(BH4)2 mixtures. The theoretically predicted Ca(NH2)(BH4) is stable [mixing energy: ∼ − 12 kJ/(mol Ca)] relative to its constituent phases [Ca(NH2)2 and Ca(BH4)2]. Since Mg(NH2)(BH4) and Ca(NH2)(BH4) have identical stoichiometry, it is interesting to examine if the prototype crystal structure of Ca(NH2)(BH4) could be a new low-energy structure in Mg(NH2)(BH4). Using the Ca(NH2)(BH4) crystal structure as a prototype, replacing Ca atomic positions by Mg, the Ca(NH2)(BH4) prototype structure is 25 kJ/(mol cation) higher in energy than the PEGS + DFT theoretically predicted Mg(NH2)(BH4) structure. On the other hand, using the Mg(NH2)(BH4) prototype structure in Ca, the prototype structure is 26 kJ/(mol cation) higher in energy than the previous PEGS + DFT theoretically predicted Ca(NH2)(BH4) structure. These comparisons indicate that, although Ca and Mg have the same ionic charge (+2e), they do not share the same lowest-energy crystal structure. In the theoretically predicted Mg(NH2)(BH4) structure, the distances of Mg−N and Mg−B are ∼2.07 and 2.47 Å, which are similar to those in Mg(NH2)2 (Mg−N: ∼2.10 Å) and Mg(BH4)2 (Mg−B: 2.41 Å), respectively. The bond lengths of N−H (1.023 Å) and B−H (1.225 Å) in Mg(NH2)(BH4) are nearly the same as those in Mg(NH2)2 (N−H: 1.025 Å) and Mg(BH4)2 (B−H: 1.221 Å) as well. In Mg(NH2)2 and Mg(BH4)2 (Figure 1), each Mg atom has four neighboring [NH2]− or [BH4]− anion units, each with the same Mg−4N or Mg−4B interatomic distances, which corresponds to Mg siting in the middle of a N or B tetrahedron. In our theoretically

Figure 1. Schematic view of Mg(NH2)2, Mg(BH4)4, and Mg(NH2)(BH4) crystal structures from ref 14, ref 39, and our PEGS + DFT predictions, respectively. Each cation atom (Mg) has four neighboring anion groups: Mg with 4[NH2] in Mg(NH2)2, Mg with 4[BH4] in Mg(BH4)2, and Mg with 2[BH4] + 2[NH2] in Mg(NH2)(BH4). Blue spheres, gray spheres, and green spheres represent Mg, B, and H atoms, respectively.

Figure 2. Static (T = 0 K) mixing energies of theoretically predicted low-energy Mg(NH2)2/Mg(BH4)2 quaternary compounds at different mixing ratios: Mg2(NH2)3(BH4), Mg3(NH2)4(BH4)2, Mg(NH2)(BH4), Mg3(NH2)2(BH4)4, and Mg2(NH2)(BH4)3. Positive and negative mixing energies (ΔEmix in eq 3) indicate unstable and stable Mg(NH2)2/Mg(BH4)2 mixed phases, respectively, relative to its constituent phases [Mg(NH2)2 and Mg(BH4)2].

the phase with a mixing ratio of Mg(NH2)2/Mg(BH4)2 = 1:1 (Figure 2), Mg(NH2)(BH4), has a static (without vibrational contributions) mixing energy of −9.8 kJ/(mol Mg). At finite temperatures (such as the temperature used in the experiments), phonons may affect structural stability. We can account for such vibrations by adding the energy associated with the phonon modes. Including vibration effects, the mixing free energy of Mg(NH2)(BH4) at finite temperatures is only changed by ∼1 kJ/(mol Mg). The negative mixing energy

Table 2. PEGS + DFT Predicted Mg(NH2)(BH4) Crystal Structure system

atom

Wyckoff

x

y

z

Mg(NH2)(BH4) (Ima2) a = 8.679 Å b = 5.915 Å c = 6.115 Å

Mg B N H H H

4b 4a 4b 8c 8c 8c

0.250 00 0.000 00 0.250 00 0.491 14 −0.389 01 0.343 06

0.348 61 0.000 00 0.055 55 0.332 33 0.473 41 −0.044 78

0.204 77 −0.193 25 0.382 24 0.190 79 0.427 81 0.349 40

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Table 3. Comparison of Experimental (Exp.) and PEGS + DFT Predicted (PEGS) Mg(NH2)(BH4) Structures: the Space Group, the Number of Formula Units in the Unit Cell, the Bond Lengths (in Å), and the Static DFT Energy Difference between PEGS exp. Experimental and PEGS + DFT Predicted Structures [ΔET=0K static = Estatic − Estatic, in kJ/(mol f.u.)] 34

Exp. PEGS + DFT

space group

formula unit

Mg−B

Mg−N

B−H

N−H

ΔET=0K static

I41 Ima2

8 4

2.52 2.47

2.06 2.06

1.22 1.22

1.03 1.02

0 1.3

predicted Mg(NH2)(BH4) structure (Figure 1), each Mg atom still has four anion neighbors, but they are 2[NH2]− + 2[BH4]− rather than 4[NH2]− or 4[BH4]−. Since the Mg−B and Mg−N interatomic distances are different, Mg sits in the middle of a distorted 2N + 2B tetrahedron. Vibrational Properties of Mg(NH2)2/Mg(BH4)2. The Mg(NH2)2/Mg(BH4)2 mixture was synthesized by ball milling a 1:1 molar ratio of Mg(NH2)2 and Mg(BH4)2. Both in situ variable temperature and room temperature X-ray diffraction (XRD) measurements were performed on the as-prepared Mg−B−N−H mixture. No discernible crystalline phases were evident in any of the collected data (i.e., reactants and products were amorphous). Future studies aimed at structural characterization (e.g., NMR) are necessary to definitively identify reactant/product phases. Vibrational measurements (PAS IR) of the resultant mixed phase (red line, Figure 3c) were

subsequently performed. The experimental PAS IR data for the mixture was compared to that of the individual reactants, Mg(NH2)2 and Mg(BH4)2 (red lines, Figure 3a and b). Finally, the experimental PAS IR measurements were compared to the DFT calculated phonon density of states of Mg(NH2)2, Mg(BH4)2, and Mg(NH2)(BH4) (black lines, Figure 3). The DFT calculated Mg(NH2)2 and Mg(BH4)2 phonon density of states (pDOS) are in good agreement with the peak positions from the experimental PAS IR measurements. In each compound [Mg(NH2)2 and Mg(BH4)2], analysis of the eigenvectors/eigenvalues obtained via direct diagonalization of the dynamical matrix reveals two distinct stretching and bending peaks (from DFT pDOS): N−H stretching and bending at ∼3200 and ∼1550 cm−1, respectively, and B−H stretching and bending at ∼2300 and ∼1250 cm−1, respectively. The N−H stretching (bending) frequency is clearly separated from that of B−H stretching (bending). In the experimental PAS IR measurements of the mixed Mg(NH2)2/Mg(BH4)2 compound, we see that the N−H and B−H stretching and bending are still present. Even though the peak positions of N− H and B−H vibrations in the mixed phase are close to those in the separated phases, the N−H vibrations have a slight blue shift (red dotted lines in Figure 3) and the B−H vibrations have a slight red shift (red dashed lines in Figure 3), which are in good agreement with the shorter (stronger) N−H and longer (weaker) B−H bonds in MgNH2BH4 than those in Mg(NH2)2 and Mg(BH4)2, respectively. We also compare the DFT calculated pDOS of the PEGS + DFT predicted quaternary Mg(NH2)(BH4) compound with the experimental PAS IR of the Mg(NH2)2/Mg(BH4)2 mixture. We find that our DFT calculated pDOS is in good agreement with the peak positions from the experimental PAS IR measurements, and also captures the blue shifts in the [NH2]− (black dotted lines in Figure 3) and the red shifts in [BH4]− (black dashed lines in Figure 3). These shifts indicate a different bonding environment for [NH2]−/[BH4]− with Mg compared to its separated phases, as we described in the previous section. This vibrational agreement suggests that a new phase is formed in the ball milling of Mg(NH2)2 and Mg(BH4)4, and the vibrations of the ball-milled phase are qualitatively consistent with those of Mg(NH2)(BH4). Dehydrogenation of Mg(NH2)2/Mg(BH4)2. TPDMS measurements for hydrogen desorption from the Mg(NH2)2/ Mg(BH4)2 compounds are given in Figure 4. On the basis of this data, no measurable B2H6 or BH3NH3 and no significant NH3 were detected during desorption [see measurement relative to Mg(NH2)2 in Figure 5]. The lack of significant ammonia desorption was unexpected, given that ammonia is a major decomposition product of Mg(NH2)2 (Figure 5) and has been observed in similar metal−N−B−H mixtures (e.g., Li3BN2H826 and Na2BNH632,33). The lack of ammonia desorption from the mixture is a qualitative change compared to Mg(NH2)2, demonstrating that the mixture does not behave as a physical combination of the starting reagents. The mixture also exhibits a low hydrogen desorption onset: approximately

Figure 3. Vibrational properties of Mg(NH2)2 (a), Mg(BH4)2 (b), and Mg(NH2)2/Mg(BH4)2 (c): The experimental PAS IR measurements (red lines) and corresponding DFT calculated pDOS (black lines). The structure of the Mg(NH2)2/Mg(BH4)2 mixture used to calculate pDOS in part c is from our PEGS + DFT predictions (Table 2). The theoretically calculated pDOS are in good agreement with the peak positions from experimental measurements. The red dotted and dashed lines are used to show the peak positions of N−H vibrations in Mg(NH2)2 and B−H vibrational in Mg(BH4)2 from PAS IR measurements, respectively. The black dotted and dashed lines are used to show the peak positions of N−H vibrations in Mg(NH2)2 and B−H vibrations in Mg(BH4)2 from DFT phonon calculations, respectively. 11197



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To identify the products or intermediates involved in the decomposition of Mg(NH2)2/Mg(BH4)2, a sample was dehydrogenated at 300 °C and subsequently quenched at room temperature for PAS IR analysis (red dotted line, Figure 6) and XRD measurement. As was observed for the as-prepared

Figure 4. TPDMS for Mg(NH2)2 + Mg(BH4)2, with measurements from as-synthesized Mg(NH2)2 and as-purchased Mg(BH4)2 given for reference. No significant quantities of impurities such as B2H6, BH3NH3, and NH3 were found. The mixture exhibits a low hydrogen desorption onset: approximately 180 °C compared to approximately 250 °C for Mg(BH4)2 and 300−350 °C for Mg(NH2)2.

Figure 6. Experimental PAS IR of the decomposition products for Mg(NH2)2/Mg(BH4)2 (red dotted line): decomposed at 300 °C and then quenched to 30 °C. DFT pDOS of possible GCLP predicted decomposition products: Mg(BH4)2 (green line) and Mg3BN3 (blue line). The black dashed line represents the vibrational peak position (∼1700 cm−1) of a decomposition product in the experimental PAS IR measurements. Additionally, for comparison, we also show experimental PAS IR of the Mg(NH2)2/Mg(BH4)2 mixture (the top solid red line) from Figure 3c.

Figure 5. TPDMS ammonia measurements for Mg(NH2)2 + Mg(BH4)2, with measurements from as-synthesized Mg(NH2)2 given for reference. Sample masses were approximately equal. The Mg(NH2)2 + Mg(BH4)2 mixture does not have the ammonia release that would be indicative of Mg(NH2)2 decomposition.

mixture, the dehydrogenated products were amorphous based on XRD measurements (making definitive structural characterization difficult using this method). In the experimental PAS IR data, the peaks related to N−H stretching and bending disappear, indicating that the [NH2]− unit was almost completely consumed during the dehydrogenation of Mg(NH2)2/Mg(BH4)2. Two broad peaks at ∼1000− 1500 and ∼2000−2500 cm−1 and a strong peak at ∼1700 cm−1 appear in the PAS IR decomposition product measurements. We next use DFT computations to provide information about the decomposition products. We used GCLP to determine the lowest energy decomposition pathways and possible products in the decomposition of Mg(NH2)(BH4). The GCLP results (reactions i and ii in Table 4) show that the lowest energy thermodynamically preferred decomposition path is decomposition into MgH2 and BN followed by decomposition of MgH2 (i.e., resulting in Mg and BN). These theoretically predicted thermodynamically stable decomposition pathways are in good agreement with the very recent MgNH2BH4 decomposition studies at up to 688 K.72 However, our experimentally observed H2 release amount of the first step at a low temperature (250 °C) is 3.4 wt % (Figure 4), which is

180 °C compared to approximately 250 °C for Mg(BH4)2 and 300−350 °C for Mg(NH2)2 (Figure 4). In particular, on the basis of Figure 4, the desorption process for the mixture proceeds via three steps, with the first step (at ∼180 °C) corresponding to the release of a very small amount of ammonia and a significant amount of hydrogen. The second and third steps, occurring at approximately 230 and 400 °C, respectively, are composed of pure hydrogen. The observed amount of H2 release is 3.4 wt % at 250°, corresponding to desorption before the shoulder reaction, and 8.3 wt % corresponding to desorption after the main desorption peak of about 280°. Decomposition Products. In a very recent paper,72 Noritake et al. investigated the decomposition of MgNH2BH4. The authors hypothesized the decomposition pathways are MgNH2BH4 → MgH2 + BN + 2H2 (327 °C) → Mg + BN + 3H2 (450 °C) based on the weight loss by the dehydrogenation. They also suggested unknown intermediates in the decomposition of MgNH2BH4 at low temperatures (∼200 °C). 11198



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Table 4. DFT + GCLP Predicted Lowest Energy Decomposition Pathways in the Decomposition of Mg(NH2)(BH4)a input list

sequential decomposition pathways

H2 wt %

ΔHT=0K static

ΔHT=0K ZPE

ΔHT=300K

with all phases

(i) MgH2 + BN + 2H2 (ii) Mg + BN + 3H2 (1) 1/4Mg(BH4)2 + 1/4Mg3BN3 + 1/12B3N3H6 + 7/4H2 (2) 1/19MgB12H12 + 6/19Mg3BN3 + 1/57B3N3H6 + 50/19H2 (3) 12/39Mg3BN3 + 1/13MgB9N + 3H2

7.3 11.0 6.4 9.6 11.0

5.60 24.15 29.22 32.13 33.89

−22.77 1.72 4.57 8.22 10.04

−17.87 7.75 10.37 13.88 15.83

without BN

T=0K T=300K ΔHT=0K are the calculated enthalpies at T = 0 and 300 K, including the static is the static enthalpy ignoring the zero point energy. ΔHZPE and ΔH vibrational energies. [Units: kJ/(mol H2).] a

significantly smaller than that of the GCLP predicted first reaction (reaction i in Table 4). The small amount of H2 release from our experimental measurements indicates that metastable intermediates could be formed in the decomposition of Mg(NH2)2/Mg(BH4)2. Furthermore, the DFT calculated pDOS of BN does not have the peaks at ∼1700 and ∼2000−2500 cm−1, indicating that BN cannot be the sole solid-state product in the experimental decomposition of Mg(NH2)2/Mg(BH4)2. Since BN is predicted to form thermodynamically but not observed experimentally, we hypothesize that formation of BN is kinetically limited. GCLP is designed to search for the lowest energy thermodynamically preferred decomposition reactions; however, we can use GCLP to explore metastable reaction pathways without the formation of BN by simply excluding the BN phase from the set of phases included in GCLP. The resulting metastable reaction pathways (reactions 1, 2, and 3 in Table 4) involve products such as Mg(BH4)2, Mg3BN3, B3N3H6, MgB12H12, and MgB9N phases. Our experimentally observed H2 release amount of the second step is 8.3 wt %, which is larger than that of reaction 1 (6.4 wt %) but smaller than that of reaction 2 (9.6 wt %) in Table 4. This observation suggests that the decomposition of Mg(BH4)2 is not a single, thermodynamically preferred step to MgB12H12 but instead may be a multistep kinetically controlled decomposition path, as suggested in refs 10 and 73. The theoretical pDOS of Mg(BH4)2 (green line, Figure 6) shows vibrational peaks at ∼1000−1500 and ∼2200− 2500 cm−1, which are in good agreement with the two broad peak positions (∼1000−1500 and ∼2000−2500 cm−1) from experimental PAS IR measurements. For Mg3BN3, the pDOS (blue line, Figure 6) shows a peak at ∼1700 cm−1, which suggests that the PAS IR vibrational peak at ∼1700 cm−1 could be from Mg3BN3 or some related Mg-boron-nitride phase. It is interesting to show that, although BN and MgB9N contain B−N bonds as in Mg3BN3, they do not have the vibrational peak at ∼1700 cm−1. The reasons behind the different vibrations of BN, MgB9N, and Mg3BN3 are because of the different local geometries. The eigenvectors/eigenvalues of the dynamical matrix for the structures show that the vibrations at ∼1700 cm−1 in Mg3BN3 are from the relative movement of two N and one B along the linear N−B−N unit. However, MgB9N and BN do not contain a N−B−N linear unit but instead one N binding with 3 B and zigzag N−B bonds, respectively. The good agreement between the vibrational peak position at ∼1700 cm−1 from the Mg3BN3 pDOS and PAS IR measurements of the experimental dehydrogenation products indicates the formation of linear N−B−N containing products in the dehydrogenation of Mg(NH2)2/Mg(BH4)2. From the above vibrational analysis, we suggest that the products in the decomposition of Mg(NH2)2/Mg(BH4)2 contain B−H bonds [Mg(BH4)2] and a linear N−B−N unit (Mg3BN3). Additional advanced characterization (e.g., solid state NMR) is needed to

further understand and clarify the specific reaction mechanism/ products in the decomposition of Mg(NH2)2/Mg(BH4)2. Investigating Reversibility of Mg(NH2)2/Mg(BH4)2. We examine the reversibility of the Mg(NH2)2 + Mg(BH4)2 mixture experimentally under two different conditions (Figure 7). The first test involved heating a sample to 250 °C (5 °C/

Figure 7. WDD for Mg(NH2)2 + Mg(BH4)2. Two different samples were used, the first ramping to 250 °C and the second ramping to 380 °C.

min ramp rate) at which point it was held for 2 h. The total hydrogen release was 3.6 wt % H2 (blue line, Figure 7). The sample was subsequently cooled to ambient temperature and charged for 18 h with 115 bar of UHP H2 before ramping again to 250 °C and holding for 2 h. The second test involved heating a sample to 380 °C (5 °C/min ramp rate). A total of 8.3 wt % H2 was released (red line, Figure 7). The sample was subsequently charged at 380 °C for 18 h with 140 bar of UHP H2 before quenching and ramping again to 380 °C and then holding for 2 h. For both tests, the samples did not desorb any measurable quantities of H2. We interpret this as an indication that the material was not reversible under the conditions studied. We note that the lack of reversibility in this system is consistent with our DFT calculated reaction enthalpies. The enthalpies (at 300 K) in Table 4 range from exothermic [−18 kJ/(mol H2)] to mildly endothermic [+16 kJ/ (mol H2)], depending on reaction products. This entire range of enthalpies falls outside the range of reversible reactions at modest temperatures and pressures. The low enthalpies of decomposition and, hence, the lack of reversibility are due to the high degree of stability of the reaction products (e.g., BN, Mg3BN3, etc.). The Mg−B−N−H system is not likely a good candidate for near ambient reversible storage. However, this challenging field requires the discovery of new compounds and new reactions in order to achieve such a reversible reaction. 11199



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(3) Zhang, Y.; Wang, Y.; Michel, K.; Wolverton, C. First-Principles Insight into the Degeneracy of Ground-State LiBH4 Structures. Phys. Rev. B 2012, 86, 094111-1−094111-4. (4) Hartman, M. R.; Rush, J. J.; Udovic, T. J.; Bowman, R. C.; Hwang, S. J. Structure and Vibrational Dynamics of Isotopically Labeled Lithium Borohydride Using Neutron Diffraction and Spectroscopy. J. Solid State Chem. 2007, 180, 1298−1305. (5) Li, H.-W.; Kikuchi, K.; Nakamori, Y.; Miwa, K.; Towata, S.; Orimo, S. Effects of Ball Milling and Additives on Dehydriding Behaviors of Well-Crystallized Mg(BH4)2. Scr. Mater. 2007, 57, 679− 682. (6) Lee, J. Y.; Ravnsbæk, D.; Lee, Y.; Kim, Y.; Cerenius, Y.; Shim, J.; Jensen, T. R.; Hur, N. H.; Cho, Y. W. Decomposition Reactions and Reversibility of the LiBH4Ca(BH4)2 Composite. J. Phys. Chem. C 2009, 113, 15080−15086. (7) Newhouse, R. J.; Stavila, V.; Hwang, S.-J.; Klebanoff, L. E.; Zhang, J. Z. Reversibility and Improved Hydrogen Release of Magnesium Borohydride. J. Phys. Chem. C 2010, 114, 5224−5232. (8) Ozoliņs,̌ V.; Majzoub, E. H.; Wolverton, C. First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System. J. Am. Chem. Soc. 2009, 131, 230−237. (9) Zhang, Y.; Majzoub, E. H.; Ozoliņ s,̌ V.; Wolverton, C. Theoretical Prediction of Different Decomposition Paths for Ca(BH4)2 and Mg(BH4)2. Phys. Rev. B 2010, 82, 174107-1−174107-7. (10) Zhang, Y.; Majzoub, E. H.; Ozoliņs,̌ V.; Wolverton, C.; Theoretical, C. Prediction of Metastable Intermediates in the Decomposition of Mg(BH4)2. J. Phys. Chem. C 2012, 116, 10522− 10528. (11) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Interaction of Hydrogen with Metal Nitrides and Imides. Nature 2002, 420, 302− 304. (12) Xiong, Z.; Wu, G.; Hu, J.; Chen, P. Ternary Imides for Hydrogen Storage. Adv. Mater. 2004, 16, 1522−1525. (13) Leng, H. Y.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. New MetalNH System Composed of Mg(NH2)2 and LiH for Hydrogen Storage. J. Phys. Chem. B 2004, 108, 8763−8765. (14) Sørby, M. H.; Nakamura, Y.; Brinks, H. W.; Ichikawa, T.; Hino, S.; Fujii, H.; Hauback, B. C. The crystal Structure of LiND2 and Mg(ND2)2. J. Alloys Compd. 2007, 428, 297−301. (15) Rijssenbeek, J.; Gao, Y.; Hanson, J.; Huang, Q.; Jones, C.; Toby, B. J. Crystal Structure Determination and Reaction Pathway of AmideHydride Mixtures. J. Alloys Compd. 2008, 454, 233−244. (16) Michel, K. J.; Akarzadeh, A. R.; Ozolins, V. First-Principles Study of the LiMgNH System: Compound Structures and HydrogenStorage Properties. J. Phys. Chem. C 2009, 113, 14551−14558. (17) Xiong, Z.; Hu, J.; Wu, G.; Chen, P.; Luo, W.; Gross, K.; Wang, J. Thermodynamic and Kinetic Investigations of the Hydrogen Storage in the Li-Mg-N-H System. J. Alloys Compd. 2005, 398, 235−239. (18) Wu, H. Structure of Ternary Imide Li2Ca(NH)2 and Hydrogen Storage Mechanisms in Amide-Hydride System. J. Am. Chem. Soc. 2008, 130, 6515−6522. (19) Chu, H.; Xiong, Z.; Wu, G.; He, T.; Wu, C.; Chen, P. Hydrogen Storage Properties of Li-Ca-N-H System with Different Molar Ratios of LiNH2/CaH2. Int. J. Hydrogen Energy 2010, 35, 8317−8321. (20) Ravnsbak, D.; Filinchuk, Y.; Cerenius, Y.; Jakobsen, H. J. A Series of Mixed-Metal Borohydrides. Angew. Chem., Int. Ed. 2009, 48, 6659−6663. (21) Aidhy, D. S.; Wolverton, C. First-Principles Prediction of Phase Stability and Crystal Structures in Li-Zn and Na-Zn Mixed-Metal Borohydrides. Phys. Rev. B 2011, 83, 144111-1−144111-8. (22) Wang, Y.; Zhang, Y.; Wolverton, C. First-Principles Studies of Phase Stability and Crystal Structures in Li-Zn Mixed-Metal Borohydrides. Phys. Rev. B 2013, 88, 024119-1−024119-10. (23) Janot, R.; Eymery, J.; Tarascon, J. Decomposition of LiAl(NH2)4 and Reaction with LiH for a Possible Reversible Hydrogen Storage. J. Phys. Chem. C 2007, 111, 2335−2340. (24) Eymery, J.-B.; Truflandier, L.; Charpentier, T.; Chotard, T.-N.; Tarascon, J.-M.; Janot, R. Studies of Covalent Amides for Hydrogen

Our paper demonstrates one method of uncovering new stable hydrides, namely, by combining existing classes of hydrides (amides and borohydrides) along with a combined experimental/theoretical approach. The current work provides interesting new insight and a path forward for future hydride and reaction discovery. To better understand the exact reaction pathway and to overcome kinetics, additional advanced characterization (e.g., solid state NMR) and formal catalyst development are necessary for future work.

■

CONCLUSION We have studied the crystal structure and decomposition products of mixed Mg(NH2)2/Mg(BH4)2 compounds using the combination of DFT computational approaches and experimental measurements. PEGS + DFT calculations confirm the existence of a mixed Mg(NH2)(BH4) phase, which is stable relative to its separated phases [Mg(NH2)2 and Mg(BH4)2]. These calculations suggest the existence of one or more stable mixed Mg(NH2)2/Mg(BH4)2 phases. The DFT calculated phonon density of states of Mg(NH2)(BH4) is in good agreement with the peak positions from experimental PAS IR measurements of the ball-milled Mg(NH 2 ) 2 /Mg(BH 4 ) 2 mixture, suggesting the mixture is not merely a physical mixture of the two compounds. The experimentally measured dehydrogenation temperature of the mixed Mg(NH2)2/Mg(BH4)2 phase is lower than that of Mg(NH2)2 or Mg(BH4)2, which further confirms that it is not simply a physical mixture of Mg(NH2)2 and Mg(BH4)2. From a combination of DFTGCLP calculations and PAS IR measurements of dehydrogenated samples, we identify the existence of B−H bonds [Mg(BH4)2] and a linear N−B−N unit (Mg3BN3) in the decomposition of Mg(NH2)2/Mg(BH4)2. Experimental desorption measurements reveal that the Mg(NH2)2/Mg(BH4)2 mixed phase is irreversible, consistent with DFT calculated enthalpies in the range of −18 to +16 kJ/(mol H2), too low for near ambient reversibility. Additional advanced characterization (e.g., solid state NMR) is needed to further understand and clarify the specific reaction mechanism/products in the decomposition of Mg(NH2)2/Mg(BH4)2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-847491-4180. Fax: +1-847-491-7820. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the U.S. Department of Energy under Grant No. DE-FC3608GO18136. The authors also thank E. Majzoub and V. Ozolins for providing the PEGS code.

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REFERENCES

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