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Steffen R. H. Jensen , Mark Paskevicius , Bjarne R. S. Hansen , Anders S. Jakobsen , Kasper T. Møller , James L. White , Mark D. Allendorf , Vitalie ...
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Phase Diagram for the NaBH4−KBH4 System and the Stability of a Na1−xKxBH4 Solid Solution Steffen R. H. Jensen,† Lars H. Jepsen,† Jørgen Skibsted,‡ and Torben R. Jensen*,† †

Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark ‡ Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University Langelandsgade 140, 8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: The binary system NaBH4−KBH4 has been systematically investigated by several complementary techniques, including in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis, and in situ 11B and 23Na magic-angle spinning (MAS) NMR. Full solubility in the system NaBH4−KBH4 and formation of a solid solution, Na1−xKxBH4, 0 < x < 1, upon heat treatment, T > 110 °C, are observed. The Na1−xKxBH4 solid solution is most clearly observed by 11B MAS NMR, which reveals a binomial distribution of Na+ and K+ ions over the six metal sites in the first coordination sphere of the BH4− unit, i.e., seven almost equally distinct peaks in the range −36.4 to −42.6 ppm with a systematic change of 1.0 ± 0.1 ppm toward lower frequency for each replacement of K+ by Na+. The solid solution Na1−xKxBH4 is metastable at room temperature, and 75% of Na0.5K0.5BH4 has separated into NaBH4 and KBH4 after 24 h at ∼24 °C, as observed by in situ 23Na and 11B MAS NMR. The composition 0.682NaBH4−0.318KBH4 has the lowest melting point of T = 458 °C, which gives rise to a lower hydrogen release temperature compared to the reactants, possibly due to enhanced kinetics. A phase diagram for the system NaBH4−KBH4 has been established, summarizing the results in a concise manner.

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INTRODUCTION

elements, have photophysical, electronic, and hydrogen storage properties and new counterintuitive structural trends.14 Some of the mixed metal borohydrides are eutectic melting at lower temperatures than the pristine metal borohydrides,15 e.g., 0.68LiBH4−0.32CaBH4 (Tm ∼ 200 °C)16 and 0.62LiBH4− 0.38NaBH4 (Tm ∼ 220 °C).17 Recently, the melting point for the eutectic mixture, 0.725LiBH4−0.275KBH4, has been reported to be as low as 107 °C.18 On the other hand, contradicting results have been reported for the composite NaBH4−KBH4, since a bimetallic compound NaK(BH4)2 was reported to form by ball-milling a 0.5NaBH4−0.5KBH4 mixture in 2009,19 while formation of a solid solution, Na1−xKxBH4, was reported in 1971.17 A solid solution is expected to form for NaBH4−KBH4 blends since both compounds crystallize with the rock-salt type structure in the cubic space group Fm−3m and the metal ions have identical charge. However, the unit cell volume of KBH4 is ∼30% larger than the volume of NaBH4. In this work, we present a systematic characterization of the formation and stability of solid solutions of Na1−xKxBH4 0 < x < 1, formed in the binary NaBH4−KBH4 system using thermal treatment, ball-milling, in situ 11B and 23Na MAS NMR

Energy storage is vital for a society based on highly fluctuating renewable energy sources. Hydrogen is a promising energy carrier owing to its benign properties and high energy content which may play an important role in future sustainable energy systems. Hydrogen can be chemically stored in the solid state as hydrides, and metal borohydrides based on the nonspherical tetrahydroborate anion, BH4−, have received significant attention owing to their high gravimetric and volumetric hydrogen contents.1−4 However, hydrogen release and uptake are often hampered by poor thermodynamic and kinetic properties and occur at high temperatures, often above 300 °C, which is not practical for mobile hydrogen storage applications. In this context, a variety of bimetallic and some trimetallic borohydrides have been discovered and investigated.5−8 In addition to their promising properties as hydrogen storage materials, metal borohydrides have a huge structural flexibility giving rise to a range of other properties.9 For example, LiM(BH4)3Cl (M = La, Gd, and Ce) show high lithium ion conductivity even at room temperature (RT)10,11 and Eu(BH4)2·2THF is fluorescent,12 while the γ-Mg(BH4)2 polymorph is a nanoporous metal hydride, which can physisorb smaller molecules, including hydrogen.13 A series of 30 new perovskite-type complex hydrides, some containing rare-earth © 2015 American Chemical Society

Received: October 8, 2015 Revised: November 18, 2015 Published: November 18, 2015 27919

DOI: 10.1021/acs.jpcc.5b09851 J. Phys. Chem. C 2015, 119, 27919−27929

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The Journal of Physical Chemistry C Table 1. Overview of Synthesis Methods and Obtained Products for the Investigated Samples

a

sample name

xNaBH4

(1 − x)KBH4

synthesis

products

s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12 s13 s14 s15 s16 s17 s18 s19 s20 s21 s22 s23 s24 s25 s26 s27 s28

0.10 0.10 0.30 0.30 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.70 0.70 0.90 0.90 1 0.90 0.80 0.70 0.682 0.65 0.60 0.50 0.40 0.30 0.20 0.10 0

0.90 0.90 0.70 0.70 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.30 0.30 0.10 0.10 0 0.10 0.20 0.30 0.318 0.35 0.40 0.50 0.60 0.70 0.80 0.90 1

ball-milled 120 min s1 + thermal treatment, 400 °C ball-milled 120 min s3 + thermal treatment, 400 °C cryo-milled 30 min s5 + thermal treatment, 400 °C s5 + thermal treatment 150 °C ball-milled 24 h (short breaks) s8 + thermal treatment, 175 °C s8 + thermal treatment, 190 °C s8 + press (1 GPa) 120 min ball-milled 120 min s12 + thermal treatment, 400 °C ball-milled 120 min s14 + thermal treatment, 400 °C manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding manual grinding

NaBH4 + KBH4 Na0.1K0.9BH4 NaBH4 + KBH4 Na0.3K0.7BH4 NaBH4 + KBH4 Na0.5K0.5BH4 Na1‑xKxBH4 NaBH4 + KBH4a Na1‑xKxBH4 Na1−xKxBH4 NaBH4 + KBH4 NaBH4 + KBH4 Na0.7K0.3BH4 NaBH4 + KBH4 Na0.9K0.1BH4 NaBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 NaBH4 + KBH4 KBH4

Less than 1% of Na0.5K0.5BH4 may be present

Characterization. In-House Powder X-ray Diffraction (PXD). PXD patterns of the as-prepared samples were measured in-house on a Rigaku Smart Lab diffractometer using a Cu source and a parallel beam multilayer mirror (Cu Kα1 radiation, λ = 1.540 593 Å, Cu Kα2 radiation, λ = 1.544 414 Å). Data were collected in the 2θ range 8°−60° at 5°/min using a Rigaku D/ tex detector. All samples were mounted in a glovebox in 0.5 mm glass capillaries sealed with glue. Fourier Transform Infrared Spectroscopy (FTIR). Samples s2, s6, s15, s16, and s28 were characterized by infrared absorption spectroscopy using a NICOLET 380 FT-IR from Thermo Electron Corporation. The samples were exposed to air for approximately 10 s prior to the data collection. Synchrotron Radiation Powder X-ray Diffraction (SR-PXD). In situ SR-PXD experiments were performed at the beamline I711 at the MAX-II synchrotron in the research laboratory MAX-lab, Lund, Sweden, with a MAR165 CCD detector system. The samples were packed in a sapphire (Al2O3) singlecrystal tube (1.09 mm o.d., 0.79 mm i.d.) using a specially designed sample holder.20 The in situ SR-PXD measurements were performed for s19 and s21 from RT to 600 °C (ΔT/Δt = 5 °C/min), while in situ SR-PXD data were collected for s3 and s14 from RT to ∼395 °C (ΔT/Δt = 15 °C/min) and kept at ∼395 °C for 15 min before being heated further to 550 °C (ΔT/Δt = 2 °C/min). The in situ SR-PXD measurement for s20 was performed from RT to 300 °C (12 °C/min), and the sample was heated further to 465 °C (ΔT/Δt = 3 °C/min), while the in situ SR-PXD measurement for s5 covered a range from RT to 500 °C (5 °C/min). All experiments were conducted in an argon atmosphere with a selected wavelength,

spectroscopy, and in situ synchrotron radiation powder X-ray diffraction.

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EXPERIMENTAL SECTION

Sample Preparation. The chemicals NaBH4 (99.99%, Sigma-Aldrich) and KBH4 (98%, Sigma-Aldrich) were used without further purification. An overview of the prepared samples is given in Table 1. Mixtures of xNaBH4−(1 − x) KBH4, x = 0.1, 0.3, 0.7, and 0.9, denoted s1, s3, s12, and s14, were mechanically treated using a Fritz Pulverisette 6 planetary ball-mill. The powders and tungsten carbide balls (balls to sample mass ratio 1:20) were loaded in a tungsten carbide vial (80 mL) under inert conditions. The powders were ball-milled at 300 rpm for 5 min intervened by 2 min break, and this sequence was repeated 24 times, resulting in a total milling time of 120 min. The mixture of 0.5NaBH4−0.5KBH4 was ballmilled for 15 min intervened by 1 min break repeated 96 times giving a total milling time of 24 h (s8). Additionally, 0.5NaBH4−0.5KBH4 was cryo-milled using a Spex 6770 Freezer/Mill for 2 min intervened by 2 min break repeated 15 times, giving a total milling time of 30 min (s5). A fraction of s8 was compressed at 1.0 GPa for 2 h to form a pellet (s11). Samples s2, s4, s6, s13, and s15 were prepared by heating s1, s3, s5, s12, and s14, respectively, to 400 °C and kept at this temperature for 20 h in an argon atmosphere in a stainless steel autoclave. Additionally, the samples s5 and s8 were heated to 150, 175, and 190 °C using the same setup (s7, s9, and s10). The samples s16−s28 were prepared by manually grinding the mixtures in a mortar for 5 min. 27920

DOI: 10.1021/acs.jpcc.5b09851 J. Phys. Chem. C 2015, 119, 27919−27929

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The Journal of Physical Chemistry C λ = 0.9938 Å. The obtained raw images were transformed into 2D powder diffraction patterns using the FIT2D program,21 which was also used to remove diffraction spots from the singlecrystal sapphire tube used as sample holder. The PXD data have been refined by the Rietveld method in the program Fullprof.22 Thermal Analysis and Mass Spectroscopy. Differential scanning calorimetry (DSC) was performed using a PerkinElmer STA 6000 apparatus simultaneously with mass spectrometry (MS) analysis of the residual gases using a Hiden Analytical HPR-20 QMS sampling system. The samples (approximately 1 mg) were placed in an Al2O3 crucible and heated from 30 to 500 °C (5 °C/min) in an argon flow of 20 mL/min. The released gas was analyzed for hydrogen and diborane. Temperature-Programmed Photographic Analysis. Approximately 10 mg of the s16−s28 samples was sealed under argon in a glass tube and placed in a home-built aluminum heating block as described previously.15 The samples were heated from RT to 480 °C (ΔT/Δt = 5 °C/min), while photos of the samples were collected every 5 s. Solid-State 11B and 23Na Magic-Angle Spinning (MAS) NMR. The 11B and 23Na MAS NMR spectra were obtained on a Varian Direct-Drive VNMRS-600 spectrometer (14.1 T) using a home-built CP/MAS NMR probe for 4 mm o.d. rotors. The spectra employed a 0.5 μs excitation pulse for 11B and 23Na rf field strengths of γB1/2π ≈ 60 kHz, a 10 s relaxation delay, and 1 H decoupling (TPPM, γB2/2π = 50 kHz). The experiments were performed at ambient temperature using airtight endcapped zirconia rotors packed with the samples in an argonfilled glovebox. Isotropic 11B and 23Na chemical shifts are relative to neat F3B·O(CH2CH3) and a 0.1 M aqueous solution of NaCl, respectively. The in situ experiments were acquired as an array of spectra, each corresponding to 15 min (88 scans with a 10 s relaxation time).

Figure 1. PXD data measured for cryo-milled 0.5NaBH4−0.5KBH4 (s5) and for xNaBH4−(1 − x)KBH4, x = 0.1, 0.3, 0.5, 0.7, and 0.9 after heat treatment (s2, s4, s6, s13, and s15). Symbols: (■) NaBH4; (●) KBH4; unmarked reflections are assigned to the solid solution Na1−xKxBH4 (λ = 1.540 56 Å).

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RESULTS AND DISCUSSION Synthesis of Na1−xKxBH4 and Initial Phase Analysis. Mechanical treatment, ball-milling, of 0.5NaBH4−0.5KBH4 is reported to result in a physical mixture of the starting materials and a small fraction of a bimetallic compound NaK(BH4)2, observed by weak reflections assigned to this compound in the PXD pattern.19 In the present work, a mixture of 0.5NaBH4− 0.5KBH4 (s5) has been cryo-milled for 30 min, providing a mechanical mixture of the starting materials rather than a new compound. Subsequently, s5 was heated to 400 °C for 20 h and cooled to RT (s6). This results in the disappearance of all Bragg reflections from the starting materials, NaBH4 and KBH4, and a new set of reflections is observed (Figure 1). The PXD data indicate that a solid solution is formed rather than a bimetallic compound, since all the new reflections are positioned between the reflections from NaBH4 and KBH4. Therefore, mixtures with different compositions of xNaBH4− (1 − x)KBH4 (x = 0.1, 0.3, 0.7, and 0.9) were mechanically treated and subsequently heated to 400 °C for 20 h (s2, s4, s13, and s15; see Table 1). The corresponding PXD data unambiguously reveal that solid solutions of Na1−xKxBH4 are formed in the entire composition range 0 < x < 1 (Figure 1). Unit cell volumes (V) extracted by Rietveld refinement of the PXD data divided by the number of formula units per unit cell (Z) are plotted as a function of the sample composition x in Figure 2 and compared with the values for the reactants, NaBH4 and KBH4. For all solid solutions, the V/Z values are

Figure 2. Unit cell volumes (V) extracted by Rietveld refinement of PXD data collected at RT divided by the number of formula units (Z) plotted as a function of composition x for Na1−xKxBH4. The uncertainties of V/Z are less than 0.01%, i.e., significantly smaller than the size of the symbols.

slightly above the line connecting the values for the two reactants, NaBH4 and KBH4, i.e., a positive deviation from Vegard’s law. Thus, the solid solutions Na1−xKxBH4 have a volume slightly larger than the volume of the sum of (1 − x)NaBH4 + xKBH4, denoted ∑V1−xNaBH4 + VxKBH4. This indicates that the Na1−xKxBH4 solid solution is not produced as a result of the compression induced by mechanical treatment23 but that it is formed due to thermal expansion of the reactants during heat treatment. To verify this indication, the 0.5NaBH4−0.5KBH4 sample was ball-milled for 24 h (s8), which resulted in a sample with PXD diffraction peaks for NaBH4 and KBH4 in addition to weak reflections ( 300 °C. The Bragg reflections from KBH4 continuously move to higher 2θ angles at T > 300 °C (Figure 9b), which suggests that the solid solution Na1−xKxBH4 is mainly formed by dissolution of solid NaBH4 in the structure of solid KBH4. At T ∼ 380 °C, the Bragg reflections (e.g., at 2θ ∼ 16.5°) become broader, suggesting that solid solutions with varying compositions are present in the sample due to thermodynamic equilibrium not being reached. The unit cell parameters of NaBH4 and KBH4 as a function of temperature are extracted by sequential Rietveld refinement and are shown in Figure 9c. The thermal expansion of KBH4 is clearly linear with the coefficient αa(KBH4) = 9.21 × 10−5 K−1 in the temperature range RT to ∼110 °C. At higher temperatures thermal contraction appears to occur, which is likely due to dissolution of NaBH4 in KBH4. At T > ∼380 °C, the rate of dissolution increases significantly and the diffraction disappears at T ∼ 460 °C due to melting of the sample. Linear thermal expansion of NaBH4 is also observed, αa(NaBH4) = 8.53 × 10−5 K−1 in the temperature range RT to ∼100 °C, which is slightly lower than the value reported in the literature, αa(NaBH4) = 5.36(6) × 10−4 K−1.26 Interestingly, at T > ∼100 °C, a positive deviation, i.e., apparent increased thermal

Figure 9. (a) In situ SR-PXD of 0.682NaBH4−0.318KBH4 (s20) heated from RT to 300 °C (11 °C/min) and then slower to 465 °C (2.5 °C/min), while (b) shows a zoom of the 2θ interval from 13° to 20° (p(Ar) = 1 bar, λ = 0.9938 Å). (c) Unit cell parameters of KBH4, NaBH4, and Na1−xKxBH4 extracted by sequential Rietveld refinement. The uncertainty of a is significantly less than the size of the symbols. Symbols: (■) NaBH4, (●) KBH4, and (◇) Na1−xKxBH4.

expansion, is observed, which may be due to the dissolution of KBH4 in NaBH4. This suggests that initially two solid solutions exist, which may turn into one upon prolonged heating. Thus, initially NaBH4 dissolves in the larger KBH4 compound, and KBH4 dissolves in the smaller NaBH4, the latter process at a slower rate. This finding is in accord with anion-substitution studies in metal borohydrides, where the compound containing the smaller anions typically dissolves more rapidly into the compound containing the larger anions.26−30 27925

DOI: 10.1021/acs.jpcc.5b09851 J. Phys. Chem. C 2015, 119, 27919−27929

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The Journal of Physical Chemistry C

0.682. At the melting point (∼460 °C), the intensity decreases due to melting and the remaining Bragg reflections move to higher angles, reflecting the excess of NaBH4. Thermal Minima for Melting and Thermal Decomposition. Previous reports suggest eutectic melting of the binary system NaBH4−KBH4 with composition 0.682NaBH4− 0.318KBH4 at Tmp = 460 °C, which is below the melting point of the two reactants, Tmp(NaBH4) = 495 °C and Tmp(KBH4) = 606 °C.17 However, we document the formation of solid solutions prior to melting at almost constant onset and peak temperatures for the melting at ∼458 and ∼460 °C for the compositional range xNaBH4−(1 − x)KBH4, 0.4 < x < 0.9. Hence, the melting is not eutectic, but the NaBH4−KBH4 mixture has a thermal minimum with the composition 0.682NaBH4−0.318KBH4.31 Further analysis of the melting process for NaBH4−KBH4 is conducted using temperature-programmed photographic analysis (TPPA), differential scanning calorimetry (DSC), and mass spectroscopy (MS) for manually mixed samples (s16−s28) in the temperature range RT to 500 °C. Visual inspections of s16−s28 by TPPA are shown in Figure S10 while photographs of s18, s20, and s22 are presented in Figure 11. The samples are all white powders at RT and do not

In situ SR-PXD data have been measured for s5 (x = 0.5) from RT to 500 °C (Figure 10a), where unit cell parameters

Figure 10. (a) In situ SR-PXD of 0.50NaBH4−0.50KBH4 (s5) heated with 5 °C/min, p(Ar) = 1 bar, λ = 0.9938 Å. (b) Unit cell parameters of KBH4, NaBH4, and Na1−xKxBH4 extracted by sequential Rietveld refinement. The uncertainty of a is significantly less than the size of the symbols. Symbols: (■) NaBH4, (●) KBH4, and (◇) NaxK1−xBH4.

extracted from sequential Rietveld refinements are plotted in Figure 10b. For this sample, the linear thermal expansion coefficients are found to be αa(KBH4) = 7.90 × 10−5 K−1 (T = 30−100 °C) and αa(NaBH4) = 8.86 × 10−5 K−1 (T = 30−100 °C). In this sample it is clearly seen that both compounds are dissolved in each other at T > 100 °C, and at T > 225 °C only one solid solution is present with the composition Na0.5K0.5BH4. At ∼ 460 °C, the Bragg reflections move to lower angles and decrease in intensity because KBH4 is released from the solid solution. In accord, this sample (x = 0.5) has a higher concentration of KBH4 compared to the one with the lowest melting point (x = 0.682, see next section). The in situ SR-PXD data collected for xNaBH 4 − (1 − x)KBH4 s3 (x = 0.30), s21 (x = 0.65), s19 (x = 0.70), and s14 (x = 0.90, Figures S4−S7) are similar to the data for s20 and s5 (Figures 9 and 10) . For all samples, Bragg reflections from NaBH4 and KBH4 disappear during heating and broad reflections from solid solutions appear. The sample s19 (x = 0.7, Figure S6) and s21 (x = 0.65, Figure S5) have compositions similar to the one with the lowest melting point (x = 0.682). At 460 °C, all diffraction peaks disappear (see Figures S5 and S6) as a result of melting of the samples, and no diffraction is observed after the melting. The sample s14 (x = 0.9, Figure S7) contains excess of NaBH4 compared to x =

Figure 11. Temperature-programmed photographic analysis of s18, s20, and s22 heated in an argon atmosphere from RT to 480 °C (5 °C/min).

visually change up to 450 °C, while onset of melting is observed in the temperature range 455−460 °C. However, the samples are not transparent like those of LiBH4−KBH4,18 and this may be due to presence of decomposition products formed shortly after the melting, similar to the decomposition behavior of Mg(BH4)2.32 DSC data for the entire heating range for all samples are shown in Figure S8. The melting point of neat NaBH4 (s16) is observed at ∼495 °C in accord with the literature, while no thermal events are observed for KBH4 (s28) at T < 500 °C, since it melts at ∼606 °C and decomposes at even higher temperatures. For the heating of s17−s27, an endothermic event at ∼458−463 °C is observed (Figure 12a) as a result of melting, followed by a second endothermic event at ∼472 °C due to thermal decomposition. The corresponding MS signals for all mixtures (Figure S9) reveal onset temperatures of hydrogen release at 465 °C, which is at a lower temperature than for neat NaBH4. The integrated DSC peak areas in the temperature range 457−465 °C for samples s16−s28 are plotted in Figure 12b. 27926

DOI: 10.1021/acs.jpcc.5b09851 J. Phys. Chem. C 2015, 119, 27919−27929

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Figure 13. Binary phase diagram of the NaBH4−KBH4 system. The melting points of s18−s22 are estimated from TPPA data (black squares), while the melting point of KBH4 is from ref 15. The solidus line is established from DSC data (white circles), while the dashed line represents the formation temperature for the solid solutions which is drawn on the basis of in situ SR-PXD data and represented by tilted black squares. Two solid solution compositions, denoted SS1 and SS2, are formed above the dashed horizontal line and below the black triangles. Above the black triangles, one solid solution is observed, Na1−xKxBH4.

K 0 . 6 9 Na 0 . 3 1 BH 4 , K 0 . 3 4 Na 0 . 6 6 BH 4 and K 0 . 6 6 Na 0 . 3 4 BH 4 , K0.45Na0.55BH4, respectively. Thus, upon increased temperatures, the two solid solutions are approaching the composition K0.5Na0.5BH4. Na1−xKxBH4 is the only reported solid solution based on alkali metal borohydrides. Mechanical treatment of LiBH4− KBH4 produces a bimetallic LiK(BH4)2 compound, and a series of new structure types with varying compositions were recently discovered in LiBH4−MBH4 (M = Rb and Cs) systems.34 The alkali metal borohydrides NaBH4, KBH4, RbBH4, and CsBH4 all have the rock-salt structure type, so these systems may also form solid solutions. Formation of solid solutions is also known for alkaline earth metal borohydrides, i.e., Mg1−xMnx(BH4)2.35 Here, V/Z also decreases for Mg1−xMnx(BH4)2 as the amount of the smaller Mg(BH4)2 increases as observed for increasing NaBH4 content in Na1−xKxBH4. Similarly to Na1−xKxBH4, Mg1−xMnx(BH4)2 decomposes at a lower temperature than the neat compounds.

Figure 12. (a) Thermal events observed by DSC in the temperature range 450−480 °C. The samples of xNaBH4−(1 − x)KBH4 (0 < x < 1, s16−s28) are heated from RT to 510 °C (5 °C/min, argon flow 20 mL/min). (b) Integrated DSC signals for s16−s28 for the DSC peak in the temperature range 457−469 °C.

Neat KBH4 gives an integral of 0, while an increase in the integral is seen as the amount of NaBH4 is increased. The maximum integral is obtained for x = 0.682, corresponding to complete melting of the sample, in accord with the composition reported in 1971.17 The enthalpy of the melting for 0.682NaBH4−0.318KBH4 is ∼190 J/g, which is slightly lower than the values for 0.725LiBH4−0.275KBH4, ∼260 J/g, and for 0.68LiBH4−0.32Ca(BH4)2, ∼250 J/g.18,33 Phase Diagram of NaBH4−KBH4. A binary phase diagram has been constructed based on the comprehensive and systematic investigations conducted in this work (Figure 13). The phase diagram illustrates melting of 0.682NaBH4− 0.318KBH4 at 458 °C and formation of solid solutions at ∼110 °C during heating at ∼10 °C/min (see Figures 7 and 8). The solidus line in the phase diagram is established by thermal analysis (DSC), and the melting points of the samples s18−s22 have been estimated by the TPPA studies. Fractions of the sample 0.5NaBH4−0.5KBH4 (s5) have been heated to 150, 175, and 190 °C, respectively, and kept at these temperatures for 20 h (s7, s9, and s10) before naturally cooled to RT. Ex situ PXD data for s7 (Figure S11) reveal formation of two solid solutions, i.e., K0.94Na0.06BH4 and K0.11Na0.89BH4. For s9 and s10, the compositions of the solid solutions are

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CONCLUSION The binary system NaBH4−KBH4 has been systematically investigated by several complementary techniques, e.g., in situ SR-PXD, thermal analysis, temperature-programmed photographic analysis, and in situ 11B and 23Na MAS NMR. It is shown that a solid solution, Na1−xKxBH4, is formed by heating a mixture in the entire composition range for 0 < x < 1 of the starting materials to above 110 °C. This contrasts a previous study that suggested formation of a stoichiometric bimetallic compound “NaK(BH4)2” with distinct sites for Na and K. The solid solution is metastable at RT and 75% of Na0.5K0.5BH4 has converted into NaBH4 and KBH4 during 24 h at ∼24 °C, as observed by in situ 23Na and 11B MAS NMR. The NaBH4−KBH4 system melts at 458 °C at the composition 0.682NaBH4−0.318KBH4, as determined by temperature-programmed photographic analysis, in situ SRPXD, and thermal analysis. Comparison of the composition 27927

DOI: 10.1021/acs.jpcc.5b09851 J. Phys. Chem. C 2015, 119, 27919−27929

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The Journal of Physical Chemistry C 0.682NaBH4−0.382KBH4 with other eutectic compositions of mixed metal borohydrides and mixed metal chlorides shows that it follows the tendency where the salt with the lowest melting point has the largest concentration at the eutectic composition, e.g., 0.725LiBH4−0.275KBH4, 0.68LiBH 4− 0.32Ca(BH4)2, 0.59LiCl−0.41KCl, 0.72LiCl−0.28NaCl, and 0.65LiCl−0.35CaCl2.15,16,18 Finally, H2 release takes places at a lower temperature for the thermal minimum composition, 0.682NaBH4−0.318KBH4, than for the two neat materials, possibly as a result of improved kinetics.

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(9) Ley, M. B.; Jepsen, L. H.; Lee, Y.-S.; Cho, Y. W.; Bellosta von Colbe, J. M.; Dornheim, M.; Rokni, M.; Jensen, J. O.; Sloth, M.; Filinchuk, Y.; et al. Complex Hydrides for Hydrogen Storage − New Perspectives. Mater. Today 2014, 17 (3), 122−128. (10) Ley, M. B.; Ravnsbæk, D. B.; Filinchuk, Y.; Lee, Y.-S.; Janot, R.; Cho, Y. W.; Skibsted, J.; Jensen, T. R. LiCe(BH4)3Cl, a new lithiumion conductor and hydrogen storage material with isolated tetranuclear anionic clusters. Chem. Mater. 2012, 24, 1654−1663. (11) Ley, M. B.; Boulineau, S.; Janot, R.; Filinchuk, Y.; Jensen, T. R. New li ion conductors and solid state hydrogen storage materials: LiM(BH4)3Cl, M = La, Gd. J. Phys. Chem. C 2012, 116, 21267−21276. (12) Marks, S.; Heck, J. G.; Habicht, M. H.; Ona-Burgos, P.; Feldmann, C.; Roesky, P. W. [Ln(BH4)2(THF)2] (Ln = Eu, Yb) - a highly luminescent material. Synthesis, properties, reactivity, and NMR studies. J. Am. Chem. Soc. 2012, 134, 16983−16986. (13) Filinchuk, Y.; Richter, B.; Jensen, T. R.; Dmitriev, V.; Chernyshov, D.; Hagemann, H. Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species. Angew. Chem., Int. Ed. 2011, 50, 11162−11166. (14) Schouwink, P.; Ley, M. B.; Tissot, A.; Hagemann, H.; Jensen, T. R.; Smrcok, L.; Cerny, R. Structure and properties of complex hydride perovskite materials. Nat. Commun. 2014, 5, 5706−5715. (15) Paskevicius, M.; Ley, M. B.; Sheppard, D. A.; Jensen, T. R.; Buckley, C. E. Eutectic melting in metal borohydrides. Phys. Chem. Chem. Phys. 2013, 15, 19774−19789. (16) Lee, J. Y.; Ravnsbæk, D.; Kim, Y.; Cerenius, Y.; Shim, J.-H.; Jensen, T. R.; Hur, N. H.; Cho, Y. W. Decomposition reactions and reversibility of the LiBH4-Ca(BH4)2 composite. J. Phys. Chem. C 2009, 113, 15080−15086. (17) Semenenko, K. N.; Chavgun, A. P.; Surov, V. N. Interaction of sodium tetrahydroborate with potassium and lithium tetrahydroborate. Russ. J. Inorg. Chem. 1971, 16, 271−273. (18) Ley, M. B.; Roedern, E.; Jensen, T. R. Eutectic melting of LiBH4-KBH4. Phys. Chem. Chem. Phys. 2014, 16, 24194−24199. (19) Seballos, L.; Zhang, J. Z.; Rönnebro, E.; Herberg, J. L.; Majzoub, E. H. Metastability and crystal structure of the bialkali complex metal borohydride NaK(BH4)2. J. Alloys Compd. 2009, 476, 446−450. (20) Hansen, B. R. S.; Møller, K. T.; Paskevicius, M.; Dippel, A.-C.; Walter, P.; Webb, C. J.; Pistidda, C.; Bergemann, N.; Dornheim, M.; Klassen, T.; et al. In Situ X-Ray Diffraction Environments for HighPressure Reactions. J. Appl. Crystallogr. 2015, 48, 1234−1241. (21) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. Two-dimensional detector software: From real detector to idealsed image or two-theta scan. High Pressure Res. 1996, 14, 235−248. (22) Rodriguez-Carvajal, J. FULLPROF SUITE; LLB Sacley & LCSIM Rennes: France, 2003. (23) Huot, J.; Ravnsbæk, D. B.; Zhang, J.; Cuevas, F.; Latroche, M.; Jensen, T. R. Mechanochemical synthesis of hydrogen storage materials. Prog. Mater. Sci. 2013, 58, 30−75. (24) Ravnsbaek, D. B.; Sørensen, L. H.; Filinchuk, Y.; Reed, D.; Book, D.; Jakobsen, H. J.; Besenbacher, F.; Skibsted, J.; Jensen, T. R. Mixed-anion and mixed-cation borohydride KZn(BH4)Cl2: Synthesis, structure and thermal decomposition. Eur. J. Inorg. Chem. 2010, 2010, 1608−1612. (25) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. In situ solid state 11B MAS-NMR studies of the thermal decomposition of ammonia borane: mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material. Phys. Chem. Chem. Phys. 2007, 9, 1831−1836. (26) Ravnsbæk, D. B.; Rude, L. H.; Jensen, T. R. Chloride substitution in sodium borohydride. J. Solid State Chem. 2011, 184, 1858−1866. (27) Arnbjerg, L. M.; Ravnsbæk, D. B.; Filinchuk, Y.; Vang, R. T.; Cerenius, Y.; Besenbacher, F.; Jørgensen, J.-E.; Jakobsen, H. J.; Jensen, T. R. Structure and dynamics for LiBH4−LiCl solid solutions. Chem. Mater. 2009, 21, 5772−5782.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09851. Additional PXD, FTIR, in situ SR PXD, TPPA, DSC, and MS data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.R.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Innovation Fund Denmark is acknowledged for financial support to the project HyFillFast. The Danish National Research Foundation is thanked for funding to the Center for Materials Crystallography (CMC, DNRF93), and the SinoDanish Center for Education and Research (SDC) is thanked for support. We are grateful to the beamline I711 at MAXlab, Lund, Sweden, for the provision of the beamtime. Finally, the use of the facilities at the Instrument Centre for Solid-State NMR Spectroscopy, Aarhus University, sponsored by the Danish Natural Science Research Councils and the Carlsberg Foundation, is acknowledged.

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REFERENCES

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