Thermal Decomposition of Mn(BH4) - ACS Publications - American

Article pubs.acs.org/JPCC

Thermal Decomposition of Mn(BH4)2−M(BH4)x and Mn(BH4)2−MHx Composites with M = Li, Na, Mg, and Ca Elsa Roedern and Torben R. Jensen* Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: The aim of this study is to investigate the thermal decomposition and hydrogen release of salt-free manganese borohydride composites with metal hydrides and metal borohydrides of lithium, sodium, magnesium, and calcium. Combined thermogravimetry, differential scanning calorimetry, and mass spectrometry were performed to measure and analyze the released gas; in situ synchrotron radiation powder X-ray diffraction was employed to investigate the reaction mechanisms. A new high-pressure polymorph of Mn(BH4)2, denoted δMn(BH4)2, was prepared by ball milling of the halide free borohydrides in the composites Mn(BH4)2−M(BH4)x for M = Li and Na. In the reactive hydride composites, Mn(BH4)2−MHx (M = Li, Na, Mg, Ca), the formation of the more stable M(BH4)x for M = Li, Na, and Ca suppresses the release of diborane and provides higher hydrogen contents in the released gas during the decomposition.

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produce diborane.15 Thus, the hydrogen storage capacity is expected to decrease due to the loss of boron from the system during continued hydrogen release and uptake in manganese borohydride. α-Mn(BH4)2 crystallizes in a trigonal unit cell with space group symmetry P3112 and has some structural similarity to Mg(BH4)2, which has attracted significant interest as potential hydrogen storage material.16−19 These reports have motivated the present study of Mn(BH4)2 composites. The preparation of Mn(BH4)2 usually involves a metathesis reaction between MnCl2 and LiBH4 or NaBH4. This reaction can be mediated by mechanochemical or solvent-based synthesis.2,20 Mechanochemically prepared Mn(BH4)2 contains LiCl or NaCl as a byproduct, reducing the hydrogen capacity significantly. Salt-free Mn(BH4)2 could be obtained in small quantities using dimethyl sulfide for extraction or synthesis.21,22 Bimetallic MMn(BH4 ) x compounds have already been investigated for M = Li, Na, K, and Mg, but no comprehensive study is available. The product of the mechanochemical reaction between MnCl2 and LiBH4 or NaBH4 only contains crystalline LiCl or NaCl, and the crystal structure could not be determined for the proposed product containing anionic [Mn(BH4)4]2− complexes. DFT calculations suggested bidentate binding of the BH4− ligands to the Mn center.23 This coordination has been confirmed by Schouwink et al. for the crystal structures of KMn(BH4)3 and K2Mn(BH4)4 successfully prepared by mechano-chemistry.21 A solid solution of MnxMg1−x(BH4)2, x = 0.2−1.0, is reported to form conserving

INTRODUCTION Energy storage remains a bottleneck in the transition to renewable energy sources, despite extensive research efforts on new materials and concepts. Hydrogen storage materials with high gravimetric and volumetric energy capacities can potentially replace gasoline.1 In the search of potential candidates, metal borohydrides draw attention due to their high hydrogen densities, but are often hardly reversible.2,3 In the 1970s, metallic hydrides were designed by combining a metal that readily absorbs hydrogen with a nonhydride forming metal, which often provides a new intermetallic with improved thermodynamic properties.4 A similar approach discovered in the early millennium, by Chen, Vajo, Dornheim, and coworkers, takes advantage of reactive hydride composites (RCH), which react exothermic and form a new dehydrogenated state.5−8 In particular for metal borohydrides, the experimental dehydrogenation temperature was found to correlate with the Pauling electronegativity of the metal coordinating directly to the borohydride complex anion.9−14 This paper explores new approaches for thermodynamical tailoring of the hydrogen storage properties of manganese borohydride and derived composites. Manganese borohydride, Mn(BH4)2, is an interesting candidate for hydrogen storage, due to high stability at room temperature, low decomposition temperature of 130−180 °C, high theoretical hydrogen capacity of 9.5 wt % H2, and the high abundance of manganese in the lithosphere. A drawback for possible utilization of Mn(BH4)2 is that the released gas during decomposition besides hydrogen is reported to contain some diborane. The mechanism for the decomposition remains not fully understood, in particular the possible side reactions that © 2014 American Chemical Society

Received: August 1, 2014 Revised: September 17, 2014 Published: September 17, 2014 23567

dx.doi.org/10.1021/jp507770g | J. Phys. Chem. C 2014, 118, 23567−23574

The Journal of Physical Chemistry C

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the trigonal structure of α-Mn(BH4)2, due to the similar ionic radii of Mn2+ and Mg2+.24 The present research aims at investigating reactive hydride composites of manganese borohydride and metal hydrides or borohydrides of lithium, sodium, magnesium, or calcium. These systems have the potential to form new bimetallic borohydrides MyMnx(BH4)z (M = Li, Na, Mg, Ca), providing new mechanisms for hydrogen release. Manganese borohydride composites or new bimetallic compounds with less electronegative metals may have higher decomposition temperature, which may eliminate the release of diborane. This study combines information from thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) with in situ synchrotron radiation powder X-ray diffraction (SR-PXD) to shed light on the formation of intermediate phases and the decomposition.

Table 1. Sample Composition and Preparation Technique

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EXPERIMENTAL SECTION Sample Preparation. α-Mn(BH4)2 was synthesized from MnCl2 (anhydrous, 99.99%) and LiBH4 (95%) by a metathesis reaction in anhydrous diethyl ether, followed by extraction with dimethylsulfide, yielding a solvate, which can be desolvated at elevated temperatures (T = 110 °C) under dynamic vacuum to yield α-Mn(BH4)2 (yield: 67% after purification, based on MnCl2). α-Mg(BH4)2 was synthesized according to a literature procedure by reacting di-n-butylmagnesium with a solution of borane dimethyl sulfide complex in anhydrous toluene.25,26 Ca(BH4)2 was synthesized from CaH2 and borane triethylamine complex according to a literature method;27,28 the batch contains an impurity of CaH2. The Mn(BH4)2−MHx (M = Li, Na, Mg, Ca) samples were prepared by manually grinding together α-Mn(BH4)2 with a metalhydride to obtain a homogeneous mixture. The Mn(BH4)2−M(BH4)x (M = Li, Na, Mg, Ca) samples were prepared mechanochemically by ball milling (BM), using a Fritsch Pulverisette 6 planetary mill, argon atmosphere, tungsten carbide vials and balls, a ball to powder ratio of approximately 35:1, a speed of 400 rpm, pausing for 2 min after 5 min of milling for a total milling time of 120 min. Two samples were prepared by treatment of α-Mn(BH4)2 at high pressure or at elevated temperature to investigate polymorphism and decomposition of Mn(BH4)2. A sample of α-Mn(BH4)2 was exposed to a pressure of approximately 1.2 GPa in a steel press for 2 × 3 h, denoted s12. A sample of 100 mg of α-Mn(BH4)2 was heated to 500 °C for 5 h in Ar atmosphere and is denoted s13. An overview of the sample preparation technique and sample composition is provided in Table 1. All samples were analyzed by simultaneous thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) as well as in situ synchrotron radiation X-ray powder diffraction (SR-PXD). All handling and manipulation of the chemicals was carried out under argon atmosphere in an MBraun Unilab glovebox with a recirculation gas purification system and gas/humidity sensors (p(O2, H2O) < 1 ppm) or by using Schlenk techniques. All commercial chemicals (Sigma-Aldrich) were used as received. Thermal Analysis. The decomposition reactions in all samples were studied by combined thermogravimetry (TG), differential scanning calorimetry (DSC), and mass spectrometry (MS) of the evolved gas, using a PerkinElmer STA 6000 apparatus and a Hiden Analytical HPR-20 QMS sampling system. Samples of approximately 3 mg were placed in an Al

sample

composite

s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12

Mn(BH4)2−LiBH4 Mn(BH4)2−NaBH4 Mn(BH4)2−NaBH4 Mn(BH4)2−Mg(BH4)2 Mn(BH4)2−Ca(BH4)2 Mn(BH4)2−LiH Mn(BH4)2−LiH Mn(BH4)2−NaH Mn(BH4)2−NaH Mn(BH4)2−MgH2 Mn(BH4)2−CaH2 Mn(BH4)2

s13

Mn(BH4)2

molar ratio 1:1 1:2 1:1 1:1 1:1 1:1 1:2 1:1 1:2 1:1 1:1

sample preparation BM, 2 h, 400 rpm BM, 2 h, 400 rpm BM, 2 h, 400 rpm BM, 2 h, 400 rpm BM, 2 h, 400 rpm manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding steel press, 1.2 GPa, 2×3h 500 °C, 5 h, Ar atm

crucible and heated from RT to 500 °C at a heating rate of 5 °C/min in argon flow. The evolved gas was transported to the MS through heated tubing and analyzed for H2 (m/z = 2) and B2H6 (m/z = 26). Atomic Emission Spectrometry. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were conducted on an SPECTRO Arcos FHS12 spectrometer. In Situ Time-Resolved Synchrotron Radiation Powder X-ray Diffraction. The reaction pathway of the decomposition was further investigated using in situ synchrotron X-ray powder diffraction (SR-PXD). In situ SR-PXD data of all samples were collected during several beam times at beamline I711 at the MAX IV laboratories in Lund, Sweden, with a MAR165 CCD detector system with X-ray exposure times of 30 s and a selected wavelength of λ = 0.99190, 0.99203, 0.99242, or 0.99410 Å. The powdered samples were mounted in a sapphire (Al2O3) single-crystal tube (o.d. 1.09 mm, i.d. 0.79 mm) in an argon-filled glovebox p(O2, H2O) < 1 ppm and placed in a specially developed in situ sample cell for investigation of solid−gas reactions.29 The temperature was controlled with a thermocouple placed in the sapphire tube in direct contact to the sample. The samples were heated with a heating rate of ΔT/Δt = 5 °C/min in p(Ar) ≈ 1 bar. All raw 2D diffraction data sets were transformed to powder patterns using the FIT2D program, incorporating wavelength calibration using a standard NIST 660a LaB6 sample, and masking single-crystal diffraction spots from the sapphire sample holder.30 Uncertainties of the integrated intensities were calculated at each 2θ point by applying Poisson statistics to the intensity data, considering the geometry of the detector.31

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RESULTS AND DISCUSSION Initial Sample Characterization. High-Pressure Polymorph δ-Mn(BH4)2. All samples were initially characterized by powder X-ray diffraction (PXD). A new compound was observed in the samples Mn(BH4)2−M(BH4)x (M = Li and Na), with the highest amounts obtained by ball milling of Mn(BH4)2−NaBH4 (s3) for a total of 12 h. The new compound could also be obtained by prolonged ball milling of α-Mn(BH4)2, indicating that a new high-pressure polymorph of Mn(BH4)2, denoted δ-Mn(BH4)2, is prepared. After pressurizing α-Mn(BH4)2 in a steel press at approximately 1.2 GPa for 2 × 3 h (s12), almost pure δ-Mn(BH4)2 was obtained. δ-Mn(BH4)2 crystallizes in a tetragonal crystal system with 23568



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space group symmetry P42nm, and the cell parameters were refined to a = b = 5.5592(6), c = 6.0777(8) Å, in accordance with recent work.32 This polymorph, δ-Mn(BH4)2, appears to be isostructural to δ-Mg(BH4)2, which has been prepared by treating the porous polymorph γ-Mg(BH4)2 at high pressures (p > 2.1 GPa).26 The sample Mn(BH4)2−NaBH4 with the molar ratio 1:2 (s2) was investigated to reproduce a previously described bimetallic compound with the composition Na2Mn(BH4)4 and an unknown structure.23 PXD data revealed small amounts of δ-Mn(BH4)2 and a large amount of remaining NaBH4. Ball milling a sample of Mn(BH4)2−NaBH4 (1:1, s3) for a total of 12 h produced a larger amount of δ-Mn(BH4)2, although Bragg reflection from α-Mn(BH4)2 and NaBH4 remains visible in the obtained diffraction data for the sample. No bimetallic sodium manganese borohydride is observed in this investigation, even after long ball milling. Figure 1 compares the patterns of as-synthesized αMn(BH4)2, the new high-pressure polymorph δ-Mn(BH4)2

Figure 2. In situ SR-PXD of Mn(BH4)2 from RT to 500 °C, ΔT/Δt = 5 °C/min, λ = 0.9920 Å. Symbols: ◆, α-Mn(BH4)2; ▽, 1.

= b = 10.4461(6) and c = 10.895(1) Å, compare well to literature values of a = b = 10.4349(13) and c = 10.8354(19) Å, based on PXD data measured at RT.16 At 137 °C, α-Mn(BH4)2 decomposes and weak diffraction of the hexagonal high temperature polymorph of LiBH4 becomes visible, which is an impurity from the synthesis. In a parallel study, a bimetallic etherate, LiMn2(BH4)5·2Et2O, was discovered, which under vacuum decomposes into LiBH4 and Mn(BH4)2.33 This new compound may have been present in the synthesis products in this study as an impurity and may possibly have produced LiBH4 during decomposition. The intermediate decomposition products formed from αMn(BH4)2 in the temperature range from 140 to 400 °C are Xray amorphous, except for some weak reflections, which resemble the decomposition products of LiBH4.15,34 The peak observed at d = 13.47 Å (2θ = 4.22°) in the temperature range from 215 to 269 °C matches to the highest intensity reflection of the “phase 1” described in ref 34. Upon the decomposition of this compound at 269 °C, three new peaks at d = 3.24, 3.19, and 2.38 Å (2θ = 17.57°, 17.88°, and 24.00°) are observed in the temperature range 269−314 °C, of which the former two match well to the decomposition “phase II” in ref 15, also attributed to LiBH4 decomposition. A sample of Mn(BH4)2 also containing LiBH4 has been investigated with temperature programed photographic technique, which revealed color changes, but no visible melting process in the temperature range ∼145−180 °C.35 In this work, an unidentified crystalline compound, denoted 1, appeared at temperatures above 400 °C during decomposition of α-Mn(BH4)2; see Figure 2. To study this in more detail, a sample of α-Mn(BH4)2 heated to 500 °C for 5 h in Ar atmosphere (s13) was investigated with PXD. Reflections at d = 4.09, 2.80, 2.39, 2.35, 2.03, 1.94 Å were observed at RT and indexation was attempted, but the data quality was insufficient for structure solution. ICP-OES was used to determine the elemental sample composition, and the ratio between Mn and B was found to be 1:0.98 in s13, and 1:1.92 for the “assynthesized” sample of α-Mn(BH4)2. This observation suggests loss of about one-half of the boron in the sample during decomposition due to the release of diborane. Decomposition of Mn(BH4)2−M(BH4)x, M = Li, Na, Mg, Ca. Small amounts of δ-Mn(BH4)2 were formed during ball milling of sample Mn(BH4)2−LiBH4 (1:1, s1). The decomposition of δ-Mn(BH4)2 is observed by in situ SR-PXD (Figure 3) at 64 °C and is associated with an increase in the diffracted intensity of the α-Mn(BH4)2 reflexes. Thermogravimetric investigations reveal that no mass loss is associated with this polymorphic transition observed at 64 °C (the experimental data are presented later in this Article). Upon further heating, LiBH4

Figure 1. SR-PXD patterns of as-synthesized α-Mn(BH4)2, δMn(BH4)2 (s12), and the ball milled Mn(BH4)2−M(BH4)x, M = Li, Na, Mg, Ca samples (s1, s2, s4, s5), λ = 0.9919−0.9941 Å. Symbols: ◆, α-Mn(BH4)2; △, δ-Mn(BH4)2; ○, M(BH4)x (M = Li, Na, Mg, Ca; ↑, CaH2.

(s13), and the four ball milled Mn(BH4)2−M(BH4)x, M = Li, Na, Mg, Ca, samples (s1, s2, s4, and s5). The amount of CaH2 in sample s5 is the same before and after ball milling. Significant peak broadening is observed in the ball milled samples. The Mn(BH4)2−MHx composites did not react during manual grinding, and the used crystalline reactants are observed in PXD data measured after sample preparation at room temperature (not shown). These samples are discussed further below. In Situ Time-Resolved Synchrotron Radiation Powder X-ray Diffraction. Decomposition of Mn(BH4)2. All samples were studied by in situ SR-PXD during continuous heating to elucidate the reaction mechanism. The decomposition of αMn(BH4)2 is initially discussed for comparison to the composites investigated in this Article. The powder diffractogram of the as-synthesized α-Mn(BH4)2 at room temperature shows a well-crystalline sample and no traces of LiCl; see Figure 2. The refined unit cell parameters, a 23569



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Figure 4. In situ SR-PXD of Mn(BH4)2−Mg(BH4)2 (1:1, s4) from RT to 250 °C, ΔT/Δt = 5 °C/min, λ = 0.9924 Å. Symbols: ◆, αMn(BH4)2; △, δ-Mn(BH4)2; ×, Mg(BH4)2; □, WC.

Figure 3. In situ SR-PXD of Mn(BH4)2−LiBH4 (1:1, s1) from RT to 280 °C, ΔT/Δt = 5 °C/min, λ = 0.9924 Å. Symbols: ◆, α-Mn(BH4)2; △, δ-Mn(BH4)2; ×, o-LiBH4; ○, h-LiBH4.

Decomposition of Mn(BH4)2−MHx. The decomposition of the samples Mn(BH4)2−MHx (M = Li, Na, Ca) follows the same mechanism; the in situ SR-PXD data from Mn(BH4)2− NaH (1:2, s9), shown in Figure 5, will be discussed here. The

undergoes a phase transition from orthorhombic o-LiBH4 to a hexagonal h-LiBH4 polymorph at T = 110 °C, in accord with the literature.34,36 The decomposition of α-Mn(BH4)2 occurs in the temperature range 127−146 °C and is apparently not affected by the addition of LiBH4. The diffraction from LiBH4 vanishes upon further heating at 247 °C due to melting of LiBH4. The final decomposition product contains 1, which starts to form at T ≈ 490 °C. The decomposition of α-Mn(BH4)2 and the M(BH4)x in the Mn(BH4)2−M(BH4)x, M = Na, Mg, Ca, composites (s3, s4, s5) appears to proceed independent from each other, similar to what was observed for Mn(BH4)2−LiBH4 (1:1, s1). The in situ SR-PXD data for the samples Mn(BH4)2−NaBH4 (1:1, s3) and Mn(BH4)2−Ca(BH4)2 (1:1, s5) are shown and discussed in the Supporting Information (Figures S1 and S2). It is noteworthy that the PXD background at low diffraction angles is higher in the Ca(BH4)2 containing sample (s5) than in the others, and the diffracted intensity of α-Mn(BH4)2 starts to decrease already at T = 86 °C. α-Mn(BH4)2 and Mg(BH4)2 have been reported to form a solid solution, Mn0.5Mg0.5(BH4)2, during ball milling of 0.5MgCl2 + 0.5MnCl2 + 2LiBH4, which crystallizes in the trigonal α-Mn(BH 4 ) 2 structure. 24 The observed broad reflections hamper reliable Rietveld refinement of our diffraction data and the determination of the sample composition, but the room temperature PXD data of Mn(BH4)2−Mg(BH4)2 (1:1, s4) appear to be an overlap of both reactants and possibly a solid solution with relatively broad Bragg reflections (Figure 4). Upon heating, the decomposition of α-Mn(BH4)2 is observed to start at T ≈ 130 °C simultaneous with the formation of a new crystalline compound, which disappears at T ≈ 191 °C. The diffracted Bragg peak positions and temperature stability range of this compound correspond well to that of ε-Mg(BH4)2 that was recently reported to form during the decomposition of γMg(BH4)2.18 This observation deviates from a previous report (ref 24), which states that all Mg(BH4)2 diffraction peaks vanish simultaneously with those from the solid solution. The origin for these differences could be the presence of LiCl in the previously investigated samples and LiBH4 in the samples used in this investigation along with different mechanochemical treatment. At T > 200 °C, the only crystalline phase in the sample is the small amount of WC, originating from the used ball milling equipment, while the Mn- and Mg-containing compounds become X-ray amorphous.

Figure 5. In situ SR-PXD of Mn(BH4)2−NaH (1:2, s9) from RT to 230 °C, ΔT/Δt = 5 °C/min, λ = 0.9920 Å. Symbols: ◆, α-Mn(BH4)2; ×, NaH; “spade”, NaBH4.

two samples of Mn(BH4)2−NaH (1:1, s8) and (1:2, s9) only differ in the relative ratio of the compounds. At room temperature, diffraction from the reactants α-Mn(BH4)2 and NaH is present. The decomposition of α-Mn(BH4)2 initiates at T = 128 °C, at which temperature the onset of the formation of NaBH4 is also observed. The diffracted intensity from NaH is almost constant in the temperature range from RT to 96 °C, decreases from 96 to 160 °C, and is again constant from 160 to 325 °C. Likewise, the formation of LiBH 4 and β-Ca(BH 4 ) 2 simultaneously to the decrease of the diffracted intensity of the metal hydride is observed during the decomposition of Mn(BH4)2 in the samples containing LiH (s6, s7) and CaH2 (s11) (see Supporting Information Figures S3 and S4). The formation of LiBH4 by the reaction of B2H6 and LiH was previously demonstrated by reactive ball milling of LiH in diborane gas.37−39 The same reaction seems to occur during the thermal decomposition of the composite samples, as described in reaction scheme, eq 1. 2 2 MHx(s) + B2H6(g) → M(BH4)x (s) x x (M = Li, Na, Ca)

(1)

The decomposition of Mn(BH4)2−MgH2 (1:1, s10) was also investigated by in situ SR-PXD (Supporting Information Figure S5). In contrast to the samples containing LiH (s6 and s7) and 23570



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total of 13.2 wt %, whereby a significant portion is diborane, as detected by MS. The decomposition of Mn(BH4)2 present in sample s1 according to eq 2 corresponds to a mass loss of 14.4 wt %, which is slightly higher than the experimental value. Thus, the decomposition of Mn(BH4)2 appears unaffected by addition of LiBH4 or the formation of δ-Mn(BH4)2 observed by in situ SR-PXD. The thermal decomposition of Mn(BH4)2−NaBH4 (1:1, s3) starts at 106 °C, with the main gas release at 128 °C, which is also a mixture of hydrogen and diborane, as detected by MS. The weight loss in the temperature range from RT to 200 °C amounts to a total of 12.4 wt %, which is in accord with the calculated mass loss of 12.5 wt % of Mn(BH4)2 present in the sample according to eq 2. In contrast, the decomposition of δMn(BH4)2 at 79 °C is not correlated with any mass loss. The composite Mn(BH4)2−Mg(BH4)2 (1:1, s4) shows an observed mass loss of 10.6 wt % in the temperature range RT and 200 °C, which compares well to the calculated mass loss of 11.1 wt % according to eq 2. The MS data reveal a release of a mixture of hydrogen and diborane in the temperature range from RT to 220 °C. At least two distinct decomposition steps are observed in the MS profile, which can be correlated to the decomposition of Mn(BH4)2, directly followed by the formation and decomposition of the ε-Mg(BH4)2. Decomposition of the sample Mn(BH4)2−Ca(BH4)2 (1:1, s5) initiates already at 83 °C, and ∼4.3 wt % of mainly hydrogen is released in the temperature range between RT and 200 °C, where only very little diborane is detected. The relatively low gas release and the high background that was observed in the X-ray studies point toward a partial decomposition of the sample during the ball milling. Prolonged ball milling of a Mn(BH4)2−Ca(BH4)2 sample led to almost complete decomposition and no gas release during thermal treatment. Investigation of Metal Hydride Containing Composites Mn(BH4)2−MHx. The results from simultaneous thermogravimetry (TGA) and mass spectrometry (MS) are shown in Figure 7 and summarized in Table 3. The onset temperature of the decompositions is similar for Mn(BH4)2 and the mixtures, as revealed by DSC and TGA measurements. The decomposition reaction in the sample containing lithium hydride Mn(BH4)2− LiH (1:1, s6) initiates already at 118 °C, but is significantly slower than the decomposition of pure Mn(BH4)2. The reaction with NaH (s8), on the other hand, is fast and strongly exothermic as observed by DSC (see Supporting Information Figure S6). The decomposition of Mn(BH4)2−CaH2 (1:1, s11) onsets at the lowest temperature, starting at 104 °C. While pristine Mn(BH4)2 releases a significant amount of toxic diborane, only hydrogen gas is detected by MS for the alkali metal hydride-containing samples. The Mn(BH4)2−LiH (1:1, s6) sample releases 4.8 wt % hydrogen in the temperature range from RT to 200 °C. This observation supports that lithium hydride reacts with diborane released from Mn(BH4)2. This reaction can be described by adding reactions 1 and 2.

NaH (s8 and s9), we observed no in situ formation of more stable borohydride, for example, Mg(BH4)2, during thermal treatment of s10. The decomposition of α-Mn(BH4)2 proceeds between 100 and 121 °C, while MgH2 decomposes in the temperature range 230−330 °C to Mg. Thermal Analysis. The decomposition of Mn(BH4)2 was initially studied using combined TGA, DSC, and MS for comparison with the new composites produced in this investigation. The mass spectrometry data revealed release of B2H6 and H2 starting at 129 °C in accordance with previous reports.15 The observed mass loss of 17.6% in the temperature range RT to 200 °C exceeds by far the theoretical hydrogen capacity of ρm(Mn(BH4)2) = 9.5 wt %, due to the release of toxic diborane. This is in good agreement with the previously proposed decomposition reaction, eq 2, corresponding to a calculated mass loss of 18.1 wt %:15,35 Mn(BH4)2 (s) → Mn(s) +

4 1 B(s) + B2H6(g) + 3H 2(g) 3 3 (2)

Samples of Mn(BH4)2 have also been shown to evolve hydrogen while stored at room temperature.40 The results of the analysis by TGA and MS data for the Mn(BH4)2−M(BH4)x systems are shown in Figure 6 and are summarized in Table 2. For the Mn(BH4)2−LiBH4 (1:1, s1) system, the release of hydrogen starts at 108 °C, but the main gas release occurs at 140 °C. It is noteworthy that the onset of the hydrogen release coincides with the phase transition in LiBH4 from an orthorhombic to a hexagonal crystal structure. The weight loss in the temperature range from RT to 200 °C amounts to a

Mn(BH4)2 (s) + +

Figure 6. Thermal analysis of Mn(BH4)2 and Mn(BH4)2−M(BH4)x (M = Li, Na, Mg, or Ca) samples from RT to 500 °C, ΔT/Δt = 5 °C/ min, combining thermogravimetry (top) and mass spectrometry (bottom).

2 M(BH4)x 3x

2 4 MHx → Mn(s) + B + 3H 2 3x 3 (M = Li, Na, Ca)

(3)

The theoretical mass loss for sample s6 according to eq 3 amounts to 6.4 wt %. 23571



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Table 2. Results of Thermal Analysis of Mn(BH4)2 and Mn(BH4)2−M(BH4)x (1:1)a mass loss [wt %]

a b

composite

Tdec(onset) [°C]

RT→200 °C

200→500 °C

released gas

calc mass loss for Mn(BH4)2, RT→200 °C [wt % H2]

Mn(BH4)2 Mn(BH4)2−LiBH4 (1:1, s1) Mn(BH4)2−NaBH4 (1:1, s3) Mn(BH4)2−Mg(BH4)2 (1:1, s4) Mn(BH4)2−Ca(BH4)2 (1:1, s5)

129 108 106 117 83

17.6 13.2 12.4 10.6 4.3

4.2 6.8 4.9 9.1 6.5

H2, B2H6 H2, B2H6 H2, B2H6 H2, B2H6 H2c

18.1b 14.4b 12.5b 11.0b 9.9b

The onset of the thermal event, Tdec, is extracted from DSC data, the mass loss from TGA data, and the residual gas was analyzed by MS. Calculated mass loss for Mn(BH4)2 according to eq 2. cTrace amounts of B2H6.

the sample; the weight loss amounts to 13.9 wt %. In the Mn(BH4)2−CaH2 (1:1, s11) sample, the amount of released diborane is reduced and the weight loss in the temperature range from RT to 200 °C amounts to 6.4 wt %, which is higher than the theoretically achievable 4.7 wt %. The results of the thermal analysis of the samples with the composition Mn(BH4)2−MHx (1:1) are summarized in Table 3. The decomposition analysis by in situ SR-PXD revealed the formation of LiBH4, NaBH4, and Ca(BH4) in the respective samples, simultaneously to the decomposition of Mn(BH4)2. LiH, NaH, and CaH2 are therefore able to absorb the diborane gas, which is released according to eq 3. No diborane is detected by the MS at higher temperatures. The combination of thermal analysis and in situ SR-PXD data suggests that the decomposition of the composite samples follows different reaction schemes. In the Mn(BH4)2−M(BH4)x (M = Li, Na, Mg) samples, Mn(BH 4 ) 2 decomposes independently, according to eq 2, and the effect of the bimetallic composite is limited to a lowering of the decomposition temperature. This is suggested both by the amount of diborane gas detected and by the mass loss that is recorded during the decomposition. The ball milling of Mn(BH4)2−Ca(BH4)2 led to a partial decomposition of the sample, reducing the total gas release. The decomposition of Mn(BH4)2 in the presence of LiH, NaH, and CaH2, on the other hand, followed eq 3 and led to the in situ formation of the thermodynamically more stable metal borohydrides M(BH4)x (M = Li, Na, Ca). The reaction is fast and exothermic in case of Na and slower in the Li or Ca system, while the formation of Mg(BH4)2 was not observed during the heating of Mn(BH4)2−MgH2 (1:1, s10). This observation is in agreement with the respective thermodynamic stability of the metal borohydrides, with NaBH4 being the most and Mg(BH4)2 the least stable, which can be correlated to the electronegativity of the metal ion.11 Because of the formation of the M(BH4)x (M = Li, Na, Ca) during the thermolysis, the release of diborane gas from the sample could be suppressed. By optimizing the Mn-

Figure 7. Thermal analysis of Mn(BH4)2−MHx (M = Li, Na, Mg, Ca) samples from RT to 500 °C, ΔT/Δt = 5 °C/min, combining thermogravimetry (top) and mass spectrometry (bottom).

The composite Mn(BH4)2−NaH (1:1, s8) releases 5.8 wt % pure hydrogen in the same temperature range, RT to 200 °C, which is in agreement with the theoretical mass loss of 5.6 wt % of this sample during decomposition according to eq 3. In contrast to the alkali metal hydride composites, the Mn(BH4)2−MgH2 (1:1, s10) sample releases both hydrogen and diborane in the first decomposition step, attributed to the decomposition of Mn(BH4)2. The mass loss and diborane release remain the same relative to the Mn(BH4)2 content in

Table 3. Results of Thermal Analysis of Mn(BH4)2 and Mn(BH4)2−MHx (1:1)a mass loss [wt %]

a b

composite

Tdec(onset) [°C]

RT→200 °C

200→500 °C

released gas

calc mass loss [wt % H2]

Mn(BH4)2 Mn(BH4)2−LiH (1:1, s6) Mn(BH4)2−NaH (1:1, s8) Mn(BH4)2−MgH2 (1:1, s10) Mn(BH4)2−CaH2 (1:1, s11)

129 118 132 120 104

17.6 4.8 5.8 13.9 6.4

4.2 6.0 8.8 4.6 4.2

H2, B2H6 H2 H2 H2, B2H6 H2d

18.1b 6.5c 5.6c 5.4c 4.7c

The onset of the thermal event, Tdec, is extracted from DSC data, the mass loss from TGA data, and the residual gas was analyzed by MS. Calculated mass loss according to eq 2. cCalculated mass loss according to eq 3. dTrace amounts of B2H6. 23572



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(3) David, W. I. F. Effective Hydrogen Storage: A Strategic Chemistry Challenge. Faraday Discuss. 2011, 151, 399. (4) Dornheim, M. Tailoring Reaction Enthalpies of Hydrides. In Handbook of Hydrogen Storage; Hirscher, M., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2010; pp 187 − 214. (5) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Interaction of Hydrogen with Metal Nitrides and Imides. Nature 2002, 420, 302− 304. (6) Dornheim, M.; Eigen, N.; Barkhordarian, G.; Klassen, T.; Bormann, R. Tailoring Hydrogen Storage Materials Towards Application. Adv. Eng. Mater. 2006, 8, 377−385. (7) Vajo, J. J.; Mertens, F.; Ahn, C. C.; Bowman, R. C.; Fultz, B. Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2 Destabilized with Si. J. Phys. Chem. B 2004, 108, 13977−13983. (8) Barkhordarian, G.; Klassen, T.; Dornheim, M.; Bormann, R. Unexpected Kinetic Effect of MgB2 in Reactive Hydride Composites Containing Complex Borohydrides. J. Alloys Compd. 2007, 440, L18− L21. (9) Rude, L. H.; Nielsen, T. K.; Ravnsbaek, D. B.; Bösenberg, U.; Ley, M. B.; Richter, B.; Arnbjerg, L. M.; Dornheim, M.; Filinchuk, Y.; Besenbacher, F.; et al. Tailoring Properties of Borohydrides for Hydrogen Storage: A Review. Phys. Status Solidi 2011, 208, 1754− 1773. (10) Li, H.-W.; Yan, Y.; Orimo, S.; Züttel, A.; Jensen, C. M. Recent Progress in Metal Borohydrides for Hydrogen Storage. Energies 2011, 4, 185−214. (11) Li, H.-W.; Orimo, S.; Nakamori, Y.; Miwa, K.; Ohba, N.; Towata, S.; Züttel, A. Materials Designing of Metal Borohydrides: Viewpoints from Thermodynamical Stabilities. J. Alloys Compd. 2007, 446−447, 315−318. (12) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.; Ohba, N.; Towata, S.; Züttel, A.; Oimo, S. Correlation between Thermodynamical Stabilities of Metal Borohydrides and Cation Electronegativites: First-Principles Calculations and Experiments. Phys. Rev. B 2006, 74, 045126. (13) Grochala, W.; Edwards, P. P. Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen. Chem. Rev. 2004, 104, 1283−1316. (14) Züttel, A.; Borgschulte, A.; Orimo, S.-I. Tetrahydroborates as New Hydrogen Storage Materials. Scr. Mater. 2007, 56, 823−828. (15) Liu, R.; Reed, D.; Book, D. Decomposition Behaviour of Mn(BH4)2 Formed by Ball-Milling LiBH4 and MnCl2. J. Alloys Compd. 2012, 515, 32−38. (16) Č erný, R.; Penin, N.; Hagemann, H.; Filinchuk, Y. The First Crystallographic and Spectroscopic Characterization of a 3 D -Metal Borohydride: Mn(BH4)2. J. Phys. Chem. C 2009, 113, 9003−9007. (17) David, W. I. F.; Callear, S. K.; Jones, M. O.; Aeberhard, P. C.; Culligan, S. D.; Pohl, a H.; Johnson, S. R.; Ryan, K. R.; Parker, J. E.; Edwards, P. P.; et al. The Structure, Thermal Properties and Phase Transformations of the Cubic Polymorph of Magnesium Tetrahydroborate. Phys. Chem. Chem. Phys. 2012, 14, 11800−11807. (18) Paskevicius, M.; Pitt, M. P.; Webb, C. J.; Sheppard, D. A.; Filsø, U.; Gray, E. M.; Buckley, C. E. In-Situ X-Ray Diffraction Study of γMg(BH4)2 Decomposition. J. Phys. Chem. C 2012, 116, 15231−15240. (19) Fang, Z.-Z.; Kang, X.-D.; Wang, P.; Li, H.-W.; Orimo, S.-I. Unexpected Dehydrogenation Behavior of LiBH4/Mg(BH4)2 Mixture Associated with the in Situ Formation of Dual-Cation Borohydride. J. Alloys Compd. 2010, 491, L1−L4. (20) Hagemann, H.; Cerný, R. Synthetic Approaches to Inorganic Borohydrides. Dalton Trans. 2010, 39, 6006−6012. (21) Schouwink, P.; D’Anna, V.; Ley, M. B.; Lawson Daku, L. M.; Richter, B.; Jensen, T. R.; Hagemann, H.; Č erný, R. Bimetallic Borohydrides in the System M (BH4)2−KBH4 (M = Mg, Mn): On the Structural Diversity. J. Phys. Chem. C 2012, 116, 10829−10840. (22) Huot, J.; Ravnsbaek, D. B.; Zhang, J.; Cuevas, F.; Latroche, M.; Jensen, T. R. Mechanochemical Synthesis of Hydrogen Storage Materials. Prog. Mater. Sci. 2013, 58, 30−75.

(BH4)2:MHx ratio and possibly ball milling of the composites to improve the mixing, the release of pure hydrogen gas could be maximized. Unfortunately, there is no indication that this reaction can be reversed to obtain the less stable Mn(BH4)2 compound again.

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CONCLUSION A new polymorph of Mn(BH4)2, denoted δ-Mn(BH4)2, is formed by mechanochemical treatment of the bimetallic borohydride mixtures, Mn(BH4)2−M(BH4)2, with M = Li, Na, Ca detected by SR-PXD. Almost pure δ-Mn(BH4)2 could be obtained after pressurizing α-Mn(BH4)2 at 1.2 GPa, which reveals similarities to polymorphs of Mg(BH4)2. The in situ SRPXD data reveal that the new compound, δ-Mn(BH4)2, transforms to trigonal α-Mn(BH4)2 upon heating, without release of hydrogen or diborane. In the Mn(BH4)2−M(BH4)x (M = Li, Na, Mg) composite systems, no new compounds apart from δ-Mn(BH4)2 were observed, and the decomposition of the metal borohydrides proceeds independently, starting at slightly lower temperatures. Reactive hydride composites of Mn(BH4)2 with LiH or NaH released 4.8 and 5.8 wt % of pure hydrogen gas at T < 200 °C. The in situ formation of the thermodynamically more stable borohydrides M(BH4)x (M = Li, Na, Ca) during the decomposition was observed by SRPXD, and the thermal analysis suggests the reaction follows eq 3. This provides a way to maintain all boron in the system, because no diborane is released, and thereby addresses one major challenge in the use of metal borohydrides as hydrogen storage materials.

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ASSOCIATED CONTENT

S Supporting Information *

Decomposition and DSC data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +45 87155939. Mobile: +45 2272 1486. Fax: +45 8619 6199. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish Council for Strategic Research via the research project HyFillFast, the Danish National Research Foundation, Center for Materials Crystallography (DNRF93), and by the Danish Research Council for Nature and Universe (Danscatt). We acknowledge funding from the European Community’s Seventh Framework Programme, The Fuel Cells and Hydrogen Joint Undertaking (FCH JU), project BOR4STORE (303428). Part of this work was supported by the COST Action MP1103 “Nanostructured materials for solid-state hydrogen storage”. The access to beamtime at the MAX-II synchrotron, Lund, Sweden, in the research laboratory MAX-lab is gratefully acknowledged.

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REFERENCES

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