Article pubs.acs.org/JPCC
An Investigation into the Hydrogen Storage Characteristics of Ca(BH4)2/LiNH2 and Ca(BH4)2/NaNH2: Evidence of Intramolecular Destabilization Natchapol Poonyayant,†,‡ Vitalie Stavila,*,‡ Eric H. Majzoub,§ Leonard E. Klebanoff,‡ Richard Behrens,‡ Natee Angboonpong,†,‡ Mutlu Ulutagay-Kartin,‡ Pasit Pakawatpanurut,† Ethan S. Hecht,‡ and Joseph S. Breit∥ †
Department of Chemistry, Center for Alternative Energy, and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand ‡ Sandia National Laboratories, Livermore, California 94551, United States § Center for Nanoscience, Departments of Physics and Astronomy, and Department of Chemistry and Biochemistry, University of Missouri, St. Louis, Missouri 63121, United States ∥ Boeing Commercial Airplanes, Everett, Washington 98203, United States S Supporting Information *
ABSTRACT: We report a study of the hydrogen storage properties of materials that result from ball milling Ca(BH4)2 and MNH2 (M = Li or Na) in a 1:1 molar ratio. The reaction products were examined experimentally by powder X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS), and temperature-programmed desorption (TPD). The Ca(BH4)/LiNH2 system produces a single crystalline compound assigned to LiCa(BH4)2(NH2). In contrast, ball milling of the Ca(BH4)/NaNH2 system leads to a mixture of NaBH4 and Ca(NH2)2 produced by a metathesis reaction and another phase we assign to NaCa(BH4)2(NH2). Hydrogen desorption from the LiCa(BH4)2(NH2) compound starts around 150 °C, which is more than 160 °C lower than that from pure Ca(BH4)2. Hydrogen is the major gaseous species released from these materials; however various amounts of ammonia form as well. A comparison of the TGA/DSC, STMBMS, and TPD data suggests that the amount of NH3 released is lower when the desorption reaction is performed in a closed vessel. There is no evidence for diborane (B2H6) release from LiCa(BH4)2(NH2), but traces of other volatile boron−nitrogen species (B2N2H4 and BN3H3) are observed at 0.3 mol % of hydrogen released. Theoretical investigations of the possible crystal structures and detailed phase diagrams of the Li−Ca−B−N−H system were conducted using the prototype electrostatic ground state (PEGS) method and multiple gas canonical linear programming (MGCLP) approaches. The theory is in qualitative agreement with the experiments and explains how ammonia desorption in a closed volume can be suppressed. The reduced hydrogen desorption temperature of LiCa(BH4)2(NH2) relative to Ca(BH4)2 is believed to originate from intramolecular destabilization.
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carbon aerogels2,3 and metal−organic frameworks (MOFs)4), thus requiring operation at cryogenic temperatures. On the other hand, the metal−hydrogen chemical bonding in many complex metal hydrides, such as metal alanates, borohydrides, and amides, is typically too strong, thus requiring high temperatures to generate acceptable equilibrium H2 pressures.5−8 However, complex metal hydrides remain an attractive group of material candidates mainly because of their potential to provide high gravimetric and volumetric
INTRODUCTION
The storage of hydrogen represents a significant challenge to hydrogen-based mobile applications (both vehicles and portable equipment) due to the low gravimetric and volumetric densities with which hydrogen can be conventionally stored. As a result, common hydrogen storage methods (gaseous, liquid, or solid-state) have rendered the associated hydrogen storage systems too heavy, too large, or both for many applications.1 The difficulty in finding a suitable solid-state method for storing hydrogen lies in the strength of interaction between hydrogen and the storage media being either too weak or too strong. Physisorbed molecular hydrogen generally interacts weakly with the pores of the high surface area adsorbing materials (e.g., © XXXX American Chemical Society
Received: March 12, 2014 Revised: June 2, 2014
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dx.doi.org/10.1021/jp502516g | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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hydrogen storage densities. Therefore, recent efforts have been devoted to designing and synthesizing new complex metal hydride storage materials with tunable metal−hydrogen interactions.9 It has been known for some time that adding a second component to a metal hydride can reduce the thermodynamic requirements for hydrogen desorption. Reilly and Wiswall10 observed that the enthalpy of hydrogen desorption (ΔHdes) of MgH2 was reduced by introducing copper. Whereas pure MgH2 has ΔHdes (at 298 K) of 78.2 kJ/mol H2,10 the ΔHdes from the 3MgH2 + MgCu2 → 2Mg2Cu + 3H2 system was observed to be 72.8 kJ/mol H2, due to the formation of the thermodynamically more stable Mg2Cu compound. A similar result was found for the Ni analogue of the modification to MgH2.11 In studies of complex metal hydrides, this approach was termed “destabilization” by Vajo and co-workers.12 This type of intermolecular or interphase destabilization is the phenomenon where a dehydrogenated intermediate alloy (Mg2Cu in the example above) requires less enthalpy to form than the dehydrogenated state (Mg) of the original compound (MgH2 in the example above). The destabilization of the starting compound with respect to dehydrogenation allows hydrogen to be desorbed at lower temperatures. Destabilization of LiBH4 by MgH2 was first demonstrated by Vajo and co-workers12,13 and represents an attractive avenue for improving the hydrogen storage characteristics of complex metal hydrides. Another way to achieve lower hydrogen desorption temperature in complex metal hydrides is to design materials with hydridic−protic interactions, where positively charged H+ (protic) and negatively charged H− (hydridic) species coexist within the same material.8 For instance, such hydridic−protic interactions are believed to promote H2 release from mixed ammine−borohydride materials.14−18 Hydridic−protic interactions can be also created using mixed anion phases, as proposed by Aoki et al. in 1LiBH4/2LiNH2,19 where the H in BH4− has a δ− character and the H in NH2− has a δ+ character. Their first-principles calculations predicted a ΔHdes of 23 kJ/ mol H2. In the absence of kinetic hindrances, such an enthalpy change allows the material to release hydrogen at approximately 80 °C with the waste heat of a proton exchange membrane (PEM) fuel cell. Experiments19 were consistent with the theoretical predictions and showed that a new intermediate phase forms after ball-milling of the borohydride−amide mixture. The new material displays a desorption temperature reduced by approximately 180 °C compared with LiBH4 alone. The intermediate phase in the Li−B−N−H system was postulated to be Li3BN2H8 (1LiBH4/2LiNH2) by Pinkerton et al.20 and later identified as Li4BN3H10 (1LiBH4/3LiNH2) by Chater and co-workers21 and Filinchuk et al.22 Li4BN3H10 forms a bcc cubic lattice with discrete BH4¯ and NH2¯ groups surrounding Li+ cations.21,22 Varying the ratio of the starting borohydride and amide can lead to materials with a different composition. For instance, reacting LiBH4 and LiNH2 in a 1:1 molar ratio results in a different phase, Li2BNH6.20,23−25 This compound contains two inequivalent Li+ cations: one Li+ is surrounded by three BH4¯ groups and one NH2¯ group, while the other Li+ is coordinated by one BH4¯ and three NH2¯ groups. After the initial reports on the LiBH4/LiNH2 systems, a number of other binary compositions were investigated, including LiBH4/NaNH2,26 LiBH4/Mg(NH2)2,27 Mg(BH4)2/ LiNH2,28 Ca(BH4)2/LiNH2,29,30 Ca(BH4)2/Mg(NH2)2,31 and Ca(BH4)2/Ca(NH2)2.31
In this work, we explore whether new borohydride−amide compounds can be generated by ball milling of the Ca(BH4)2/ LiNH2 and Ca(BH4)2/NaNH2 systems and whether improved hydrogen storage properties result. Chu et al. recently investigated the Ca(BH4)2/LiNH2 system;30 however, the structure, reversibility, and detailed decomposition of this material were not reported. Here, we expand this previous work by exploring the reaction products produced by ball milling the 1:1 Ca(BH4)2/LiNH2 and Ca(BH4)2/NaNH2 systems and study their hydrogen desorption character and reversibility. In addition, we also investigate the evolution of gases other than H2 (e.g., diborane and ammonia) upon heating. We employ a combination of experimental techniques, including the versatile simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) method developed at Sandia, to gain insight into the chemical reactions occurring as these materials are heated, releasing hydrogen. We theoretically examine the 1:1 Ca(BH4)2/LiNH2 system with prototype electrostatic ground state (PEGS) calculations and apply a new theoretical approach termed multiple gas canonical linear programming (MGCLP) to determine the decomposition pathways and thermodynamics of the reaction components in the Ca−Li−B− N−H system at both virtual vacuum and desorption into 2 bar overpressure of hydrogen.
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EXPERIMENTAL SECTION Sample Preparation. Synthesis and handling of airsensitive materials were carried out under an argon atmosphere using standard Schlenk and glovebox techniques. Ca(BH4)2, LiNH2, and NaNH2 were of reagent grade and were obtained from Aldrich. Ca(BH4)2 was recrystallized from tetrahydrofuran (THF) and dried at 180 °C in vacuum (20 mTorr) for 16 h to remove the solvent. In a glovebox, Ca(BH4)2 and the appropriate amides were ground together by hand with a mortar and pestle for 5 min, then placed in a 25 mL steel grinding vial containing two tungsten-carbide balls. The vial was sealed, wrapped with tape, removed from the glovebox, and then milled on a SPEX 8000 high-energy mixer/mill for 1 h with 5 min pauses every 20 min. The reaction product samples were unloaded and weighed in the glovebox. Sample Characterization. The crystal structures of the ball-milled reaction product samples were examined by powder X-ray diffraction (XRD). The XRD patterns were collected using a rotating anode Rigaku diffractometer (RU-300) with a Cu target. Samples were loaded into glass capillaries and sealed using vacuum grease in the glovebox before insertion into the instrument. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using a Mettler TGA/DSC-1 STARe system were used to measure the decomposition characteristics. Samples were sealed in 100 μL aluminum crucibles within the glovebox, and the TGA/DSC system punctured the crucible lid just before the measurements were made. Generally, the samples were heated from 25−600 °C with a heating rate of 5 °C/min and an Ar flow rate of 20 mL/ min. STMBMS studies, providing characterization of the decomposition products, were conducted using an apparatus shown in Figure S1 (Supporting Information) and described by Behrens in detail elsewhere.32 Briefly, an STMBMS decomposition experiment is conducted in a reaction cell that is fitted with a well-characterized exit orifice that can be used to control the rate of flow of gases out of the reaction cell. Varying the size of the orifice allows the pressure of gases within the reaction cell B
dx.doi.org/10.1021/jp502516g | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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the Vienna ab-initio simulation package (VASP).34,35 Electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 600 eV. Brillouin zone integrations were performed using Monkhorst−Pack grids with a minimum of 4 × 4 × 4 k-points per reciprocal unit cell. We used projector augmented wave (PAW) GGA pseudopotentials35,36 using the exchange correlation of Perdew and Wang (PW91)37 included in the VASP package. Phonon frequencies were calculated in the harmonic approximation using the linear response capabilities in VASP5. In addition to calculating the energetics and crystal structures of possible species in the Ca−Li−B−N−H system, we also theoretically predicted the course of desorption reactions releasing hydrogen and other potential volatile species. The phase diagrams for the Ca−Li−B−N−H system were calculated at specific compositions as a function of temperature at fixed pressure in order to determine decomposition reactions and to establish the phase stability of possible reaction products. This was accomplished using the recently developed MGCLP method for calculation of the phase diagram with multiple gas species. This is related to the grand canonical linear programming (GCLP) method developed previously.38 In the GCLP method, the grand canonical ensemble is used with hydrogen as the reservoir gas. This means that the hydrogen chemical potential is set externally as a fixed parameter during the minimization and the solid phases compete against one another to determine the phase fractions. In MGCLP, we instead use the canonical ensemble and let multiple gas phases compete in the free energy minimization. In this case, the Gibbs free energy function is given by
to be controlled. The simultaneous measurement of the rate of force change (mass loss and thrust) and the mass spectra of the gases exiting the reaction cell provides the rates of formation of gas-phase compounds formed during thermal decomposition. The time-of-flight velocity spectra of the neutral gases that exit the reaction cell are used to determine whether ion signals measured with the quadrupole mass spectrometer are parent or daughter ions. Thus, ion signals at a specific mass-to-charge (m/z) value, measured with the mass spectrometer, can be associated with a compound that evolves from the reaction cell. In a typical STMBMS experiment, the sample is loaded into a reaction cell within the glovebox, fitted with a ceramic cap having a small diameter orifice, and then transported under an inert atmosphere for placement within the STMBMS instrument. The sample is subsequently heated to 700 °C while the sample mass loss and mass spectra of the evolved species are recorded at the same time. The STMBMS instrument was designed to conduct experiments that provide the identities and rates of formation of the compounds involved in the reactive processes that control the decomposition of energetic materials. The volumetric temperature-programmed desorption (TPD) and hydrogen absorption experiments were done in a Sievertstype apparatus (PCT-Pro 2000, Hy-Energy and Setaram). The desorption/decomposition was performed by heating the samples from room temperature to 400 °C, with the desorbing volatiles collected into an evacuated fixed volume. A thermocouple was placed in the center of the sample holder for accurate temperature measurements during the experiments. The hydrogen absorption experiments were performed isothermally at 230 °C under 106 bar H2 pressure. Pressure changes during the dehydriding and rehydriding of the samples were quantified with calibrated pressure transducers and recorded using a LabVIEW-based program.
G(N , V , T ) = PV +
∑ xiFi(N , V , T ) i
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where all phase fractions, xi, for solids and gases are represented in the summation. The pressure P = ∑gPg is the sum of partial pressures of each of the gases, where each partial pressure Pg = xgRT, where R is the gas constant. The Helmholtz free energy for each phase is F = E0 − TSvib, where E0 is the total electronic DFT energy and Svib is the vibrational entropy calculated within the harmonic approximation. There is an additional Sakur− Tetrode (ST) term for the gas phases, where for a monatomic ideal gas Sgas = NkB[5/2 + ln(nQ/ρ)]. We include a vibrational term in our gas phase entropy that enters in the partition function as Z = zvibN/N!, where the gas indistinguishability term N! introduces minor numerical changes in the ST equation. In our treatment, we neglect the rotational contribution to the entropy in the gas phases. The logarithm in the ST equation reduces the influence of this term for reasonable pressures, for example, 1 ×10−10 bar and above, and aside from the quantum concentration, nQ = (2πm/(βh2))3/2 containing the molecular mass, no empirical constants were used in the gas free energy. Here β = 1/(kBT), where kB is the Boltzmann constant, and h is Planck’s constant. Because the free energy of the gases is a function of the pressure, the minimization of the free energy for a given temperature T and volume V as a function of the mass fraction of the gas phases must be performed self-consistently. Because of this fixed volume, reactions may occur as the gas expands and the pressure builds. This may seem to complicate the study of desorption reactions, but this is easily handled by adjusting the atomic composition constraints as gases are produced. As the temperature is raised, phase fractions change, indicating a reaction. At each reaction point, if gases are produced, the
THEORY Theoretical studies of the reaction products produced by ball milling of Ca(BH4)2 and LiNH2 were conducted by the PEGS method developed by Majzoub and Ozolinš.33 Briefly, PEGS provides a more rigorous method to predict crystal structures by incorporating the understanding that these complex anionic materials are dominated by electrostatic interactions. Infrared and Raman vibrational spectra of existing alanates and borohydrides have established the nature of many of these compounds to be molecular ionic structures with the bending and stretching modes of the anions distinctly separated from the crystal modes involving motion of the cations. In metal− hydrogen compounds containing complex anions, the metal atoms are frequently alkali or alkaline earth, and the complex anions one of [NH]2−, [NH2]−, [BH4]−, [AlH4]−, and [AlH6]3−. By treating the complex anionic units (NH2− and BH4−) as rigid and performing a global minimization of the total electrostatic energy, one obtains a crystal structure that can then be the basis of density functional theory (DFT) calculations. The PEGS Hamiltonian is ∑i
