Article pubs.acs.org/JPCC
High Pressure Behavior of Hydrogen Storage Material Guanidinium Borohydride Guangyu Qi,† Kai Wang,† Xiaodong Li,‡ and Bo Zou*,† †
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China
‡
ABSTRACT: A combination of synchrotron powder angle-dispersive X-ray diffraction (ADXRD) and Raman spectroscopy has been utilized to study high pressure behavior of the chemical hydrogen storage material guanidinium borohydride (GBH) [C(NH2)3]+[BH4]−. A reversible structural phase transition was observed at approximately 0.4 GPa, evidenced by the obvious changes in ADXRD patterns and Raman spectra. Both Raman spectra and the first-principles calculation revealed that new dihydrogen bonds generated under high pressure, which was proposed to be the reason for the observed phase transition. This research explored the pressure induced changes of GBH and studied the evolution of dihydrogen bonds, which can give insight into the improvement of the hydrogen release properties.
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INTRODUCTION Hydrogen has been considered as the next generation green and sustainable energy source for its highest energy density by weight and its only pollution-free combustion product being water. Presently hydrogen has gained numerous applications in many aspects, especially as a mobile fuel (e.g., hydrogen powered fuel cell vehicles).1,2 BNH compounds have attracted general interest for their high hydrogen capacity.3,4 For example, ammonia borane (NH3BH3, AB) is a promising hydrogen storage material with a hydrogen storage capacity as high as 19.6 wt %, and its derivatives have been investigated intensively under ambient and extreme conditions.5−11 The dihydrogen bond, as an unconventional weak noncovalent interaction which exists widely in BNH compounds, was first discovered in transition metal hydrides, where the MH bond acts as a dihydrogen bond acceptor and the NH bond acts as a dihydrogen bond donor.12 Afterward, Crabtree et al. have found that intermolecular BH···HN interactions in BNH compounds are another kind of typical dihydrogen bonds.13 Through the structural research based on Cambridge Structural Database (CSD), they have provided characteristic metric data for this interaction: the H··· H distance is in the range of 1.7 to 2.2 Å and the NH···H group tends to be linear, while BH···H tends to be bent. Recently, the dihydrogen bond has widely achieved attention for its novel properties in BNH compounds.14,15 In the studies of AB, the network of dihydrogen bonds between amine protons and boron hydrides has been regarded as the key phenomenon that dictates the dehydrogenation of AB.16−18 And in Chen’s work, dihydrogen bonds in alkali-metal amidoboranes facilitate the dehydrogenation that alkali-metal amidoboranes can produce hydrogen more readily.19 As a BNH derivative, guanidinium borohydride (GBH) [C(NH2)3]+[BH4]−, connected by dihydrogen bond, is a © 2016 American Chemical Society
potential chemical hydrogen storage material with a hydrogen content of 13.1 wt %. In 1954, the synthesis of GBH was first published by Schechter.20 Later, further research by Titov et al. studied the additional synthetic methods, properties of GBH and showed that GBH starts to decompose at 100 °C.21 More recently, Groshens and Hollins reported the self-sustaining thermal decomposition of GBH and determined the crystal structure by single-crystal X-ray diffraction.22 It crystallizes in the tetragonal space group I41/amd with Z = 8 in a unit cell, the lattice parameters are a = b = 6.7433(8) Å, c = 24.195(3) Å. As depicted in Figure 1, the guanidinium cation and the
Figure 1. Ball-and-stick model of a one-dimensional GBH tape. The black dashed lines stand for dihydrogen bonds.
borohydride anion are connected one by one in a plane through the close dihydrogen bonding and form a tape. The low-temperature and ammonia suppression hydrogen generation of GBH was reported by adding calcium borohydride or lithium borohydride to form new dihydrogen bonds between guanidinium and borohydride.23,24 As a fundamental thermodynamic parameter, pressure is extensively used to develop a comprehensive understanding of Received: April 25, 2016 Revised: May 26, 2016 Published: June 7, 2016 13414
DOI: 10.1021/acs.jpcc.6b04154 J. Phys. Chem. C 2016, 120, 13414−13420
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to generate high pressure. A 150 μm diameter hole in the middle of the T301 steel gasket, which was preindented to the thickness of 50 μm, was used as the sample chamber. A small ruby chip was placed in the sample chamber for pressure measurement, utilizing the R1 ruby fluorescence technique.34 Argon or silicon oil were utilized as pressure transition medium (PTM). High pressure Raman spectra on GBH powder were carried out by a Raman system using a 671 nm single-mode DPSS laser excitation source (output power 20 mW). We performed Raman measurements with our indigenously developed microRaman system assembled around a spectrometer (iHR 550, Horiba Jobin Yvon) combined with a liquid-nitrogen cooled CCD (Synapse, Horiba Jobin Yvon). The resolution of the Raman system was about 1 cm−1. ADXRD experiments were conducted on BL15U1 beamline at the Beijing Synchrotron Radiation Facility (BSRF). A monochromatic 0.6199 Å wavelength beam was utilized for pattern collection. We used CeO2 as a standard sample for calibration of geometric parameters and recorded Bragg diffraction rings with a Mar345 imaging-plate detector. The FIT2D software was used to integrate the XRD patterns.35 All the experiments were carried out at room temperature. The CASTEP module in the Materials Studio program was applied to the first−principles calculation of GBH.36,37
material properties, create new materials which may have novel characteristic in chemical and physical properties.25 For instance, the thermolysis of AB under high pressure releases almost the entire hydrogen content of the molecule in two distinct steps instead of three steps at ambient pressure.26 Moreover, the relatively weak intermolecular interactions can be easily modified by external pressure. The application of high pressure by diamond anvil cell (DAC) techniques can provide precise tuning of intermolecular distances for molecular crystal and facilitate close packing, which changes the balance of intermolecular interactions that leads to new molecular reorientations. Therefore, the adoption of pressure is an ideal strategy to control the structural, electronic, and vibrational properties and has been widely used in the studies of weak intermolecular interaction bonded systems.27−31 High pressure Raman study of AB showed that NH stretching modes exhibited apparent red shift with the increasing of pressure, which gave evidence for the existence of dihydrogen bonds in AB.32,33 Consequently, high pressure can be successfully applied to investigate the dihydrogen-bonded BNH compounds. In this present research, we conducted in situ angledispersive X-ray diffraction (ADXRD) and Raman spectroscopy to study the high pressure behavior of GBH in a DAC at room temperature and pressures up to 5.0 GPa. A reversible phase transition at approximately 0.4 GPa was found in ADXRD patterns and Raman spectra. Both external and internal modes of Raman spectroscopy were obtained and gave detailed information on the motions of molecular fragments and dihydrogen bonds. We also analyzed the evolution of dihydrogen bonds with the help of density-functional theory (DFT) calculations. We found the mechanism of this phase transformation to be the generation of new dihydrogen bonds. The expectation of our study is to explore the pressure induced changes of GBH and understand the evolution of dihydrogen bonds under high pressure, which can provide guidance into the promotion of the hydrogen release properties for this material.
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RESULTS AND DISCUSSION
The variation of XRD spectra can reflect the change of crystal structure directly. The representative ADXRD patterns of GBH up to 5.0 GPa are illustrated in Figure 2. With the increase of pressure, all the diffraction peaks shifted to higher angles, indicating the reduction of unit cell volume. Above 0.5 GPa, two new peaks marked by asterisks appeared, accompanied by
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EXPERIMENTAL SECTION GBH was synthesized using the method proposed by Groshens.22 Two kinds of the raw materials were obtained commercially, namely, sodium borohydride (NaBH4, 98%, Aladdin) and guanidinium carbonate (C2H10N6H2CO3, 99%, Aladdin). GBH was synthesized by reaction of NaBH4 and C2H10N6H2CO3 in 2-propanol. A mixture of C2H10N6H2CO3 (0.005 mol) and NaBH4 (0.01 mol) in 30 mL of 2-propanol was placed in a 100 mL round-bottom flask under a nitrogen blanket. The suspension was stirred at ambient temperature (approximately 20 °C) using a magnetic stir bar. The reaction equation is as follows: C2H10N6H 2CO3 + 2NaBH4 → 2[C(NH 2)3 ]+ [BH4]− + Na 2CO3
Twenty-four hours later, the mixture was filtered, and the solid was washed twice with 5 mL 2-propanol. Then the filtrate was combined with 40 mL diethyl ether, shaken to mix fully, and cooled in a −40 °C freezer for 12 h. The colorless crystalline solid product was collected on filter paper and washed twice with 5 mL diethyl ether. After initially drying the solid with a N2 stream for 1 h, the target white compound was obtained. The purity was checked under ambient conditions by Raman spectroscopy and synchrotron X-ray powder diffraction. The symmetric DAC with 400 μm diamond culets was applied
Figure 2. Representative of ADXRD patterns at elevated pressures. The asterisks denote the appearance of new peaks, the down-facing arrows represent the decrease of the peaks. 13415
DOI: 10.1021/acs.jpcc.6b04154 J. Phys. Chem. C 2016, 120, 13414−13420
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The Journal of Physical Chemistry C the gradual disappearance of original peaks of phase I marked by down-facing arrows. Considering the differences between the diffraction patterns at 0.3 and 0.5 GPa, it is evident that the crystal structure changed, and GBH underwent a phase transformation (from phase I to phase II) at 0.5 GPa. When the pressure rose up to 1.0 GPa, another two peaks of phase II at 2θ = 10.0° and 13.4° could be clearly observed. We indexed the diffraction pattern at 1.0 GPa into a monoclinic structure. The indexed lattice parameters were a = 24.09 Å, b = 7.15 Å, c = 5.52 Å, β = 93.72°, and unit cell volume V = 947.68 Å3. With further compression to 5.0 GPa, we did not observe any new peaks, indicating that the crystal structure of phase II is stable under 5.0 GPa. Upon total release of pressure, the diffraction pattern showed that the crystal structure returned to the initial phase, suggesting the phase transition is reversible. In order to get more valuable information about the phase transition, we carried out in situ Raman spectra to explore the high pressure behavior of GBH. All of the observed Raman modes of GBH appear to stem from the comprising ions, C(NH2)3+ and BH4−, and are assigned based on the previously reported literature.38,39 In this work, Raman vibration spectra of GBH were measured as a function of pressure from 1 atm to 5.1 GPa. The selected GBH Raman spectra in the range of 70− 320, 400−1200, 1390−1810, and 2000−3600 cm−1 at the various pressures are presented in Figures 3(a), 4(a, b), 5, and
Figure 4. Pressure evolution of internal Raman spectra in the spectral range of (a) 400−1200 and (b) 1380−1820 cm−1.
condition consisted of three external modes (85, 116, and 195 cm−1). At 0.4 GPa, the intensity of these modes at 85 and 195 cm−1 (marked with down-facing arrows) decreased rapidly. Meanwhile, we also observed that the band at 116 cm−1 became sharper. All of these implied that a pressure induced phase transition of GBH began at 0.4 GPa. As depicted in Figure 3(b), the slope of the external bands changed abruptly during the phase transition. Beyond the phase transition, all Raman vibration modes in the external region exhibited normal blue shift, which indicated external mode hardening because of the enhancements in intermolecular interactions. The internal modes can provide fundamental information about the chemical environment around specific groups. In Figure 4(a, b), we present the internal Raman spectra of GBH in the ranges of 400 to 1200 cm−1 and 1390 to 1810 cm−1 at various pressures. In these regions, considerable variations in the spectrum can be observed at 0.4 GPa. The CN3 angle deformation vibration modes labeled with down-facing arrow at 522 and 529 cm−1 vanished suddenly at 0.4 GPa. At the same time, a new band marked with an asterisk at 513 cm−1 was detected and maintained up to 5.1 GPa. The selected spectra of the internal modes ranging from 1380 to 1750 cm−1 assigned to NH2 scissoring vibration is illustrated in Figure 4(b). Four Raman vibration modes (1554, 1557, 1649, and 1665 cm−1) were measured in this region. At 0.4 GPa, a new characteristic labeled with an asterisk at 1440 cm−1 has emerged. The frequency shift of these internal modes in the range of 400 to 1200 cm−1 and 1390 to 1810 cm−1 is depicted in Figure 5(a, b). In agreement with the shift of external Raman vibrations, the internal Raman peaks exhibited an obvious discontinuous shift at 0.4 GPa. Most of these internal Raman modes showed blue shift to high frequency. We attributed this blue shift to the reduced bond lengths as well as increased bond strengths with increasing pressure. After the phase transition, the NH2 scissoring mode at 1557 cm−1 during the following compression exhibited particular red shift which can be explained by the strengthen of dihydrogen bonds. We summarize the evolution of Raman spectra on BH stretching vibration modes (2100−2400 cm−1) and NH stretching vibration modes (3100−3500 cm−1) as a function
Figure 3. (a) Selected Raman spectra of external modes in the range of 70−320 cm−1 under different pressures. (b) Corresponding peak positions of the external modes as a function of pressure. The vertical lines denote the occurrence of phase transition.
6, respectively. The Raman spectrum at 0.4 GPa underwent multiple simultaneous changes, which demonstrated that GBH displays a pressure induced phase transition. The Raman spectra of phase II evolved continuously with increasing pressure, indicating the stability of this phase up to the maximum pressure (5.1 GPa). The high pressure phase II reverted to the original phase during the decompression, illustrating that the pressure induced phase transition of GBH is completely reversible. Furthermore, the Raman shifts of both the external and internal modes exhibit a discontinuity at 0.4 GPa, as depicted in Figure 3(b), 5(a, b), and 7(a, b). The Raman external vibration modes are very sensitive to external pressure due to the weak intermolecular interactions and served as an indication of structural change. Figure 3(a) shows the selected Raman spectra of the external modes at various pressures and Figure 3(b) shows the Raman shift of the external modes during compression. The spectrum at ambient 13416
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Figure 5. Frequency shift of the internal Raman modes as a function of pressure in the range of (a) 500−1050 cm−1 and (b) 1530−1680 cm−1.
BH and NH stretching modes gave strong evidence that dihydrogen bonds had an enormous variation at 0.4 GPa. The Raman shift of BH and NH stretching modes with increased pressure is depicted in Figure 7(a, b). What calls for special attention is that the BH stretching modes showed a large blue shift, however, most of the NH stretching modes did not exhibit an apparent shift. Because of the competition between dihydrogen bonds and close packing, the NH bond length did not have a distinct decrease, while the length of BH bonds reduced apparently under high pressure. And we suggest that the dihydrogen bond donor NH bonds can be influenced more prominently than the dihydrogen bond acceptor BH bonds in line with previously reported studies.32,33 Through the analysis of ADXRD experiments and Raman scattering, we confirmed the phase transition of GBH under high pressure. In Raman scattering patterns, the external modes and the stretching modes of BH and NH bonds, which can reflect the variation of dihydrogen bonds, changed abruptly in the phase transformation. Thus, we inferred that dihydrogen bonds were altered a lot during the phase transition. How the dihydrogen bonds evolve under high pressure and what is the function of dihydrogen bonds in phase transition? In order to solve the questions, we carried out first-principle calculation based on DFT. As depicted in Figures 1 and 8(a), the crystal packing is controlled by dihydrogen bonds and van der Waals interactions. GBH exhibits can be described in terms of stacks and layers of one-dimensional GBH tapes. Within the tape, the guanidinium cation and borohydride anion are held together one by one with dihydrogen bonds. The distance between the tapes is so long that the primary effect between the tapes is van der Waals interaction. From the calculated structure of GBH at 1.0 GPa in Figure 7(b), we can see that the tapes became closer with the increase of pressure. The two hydrogens on the guanidinium which do not give any effort to dihydrogen bonds at ambient pressures are involved in close dihydrogen bonding to the hydrides of the borohydride which belongs to another tape. The dihydrogen bonding networks of GBH come into being, and the principal interactions between the tapes turn into dihydrogen bonds instead of van der Waals interactions. Thus, we put forward a mechanism for this phase transition. Upon compression, the distance between GBH tapes decrease.
Figure 6. Pressure evolution of BH and NH stretching modes in the spectral range of 2000−3600 cm−1.
of pressure in Figure 6. As the BH bond is the dihydrogen bond acceptor, and NH is the dihydrogen bond donor, the change of the stretching modes on BH and NH can directly reflect the variation of the dihydrogen bond. Four BH stretching bands (2126, 2178, 2252, and 2286 cm−1) and three NH stretching bands (3262, 3364, and 3428 cm−1) were observed at 1 atm. None of these stretching modes exhibited obvious changes under low pressure range. When the pressure rose up to 0.4 GPa, BH and NH stretching regions showed apparent differences. The BH stretching modes at 2178 and 2252 cm−1 displayed an increase on their intensity. Also a new band marked with asterisk at 2351 cm −1 appeared. Furthermore, in the NH stretching region, the peak at 3428 cm−1 became more intense, and a new peak at 3329 cm−1 can be clearly detected. The original band labeled with a downfacing arrow at 3364 cm−1 gradually lost its intensity and finally disappeared into the scattering background. The changes in the 13417
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Figure 7. (a) Frequency shift of BH stretching modes as a function of pressure in the range of 2120−2430 cm−1 and (b) frequency shift of NH stretching modes as a function of pressure in the range of 3240−3460 cm−1.
the new dihydrogen bonds came into being and caused the appearance of these BH and NH stretching modes. From the frequency shift of NH2 scissoring, we can observe a red shift of this vibration mode, indicating the enhancement of dihydrogen bonds. Also in the CN3 angle deformation region, the original peaks disappeared, and a new mode at 513 cm−1 could be observed. This can be explained by the fact that the change of dihydrogen bonds lead to the chemical environment variation on the N of guanidinium. After total release of pressure, GBH returned to the initial phase I. Compared to former studies, this reversibility is attributed to the fact that without steric hindrance, the energy barrier for the phase transition is really small which can be easily overcome under the synergistic effect of dihydrogen-bonding, electrostatic, and van der Waals interactions when pressure is released.40−42 The analysis of Raman scattering and first-principles calculation showed that the formation of dihydrogen bonds played a vital role in the phase transition and resulted in structure change of GBH. As previously reported, the dehydrogenation of BNH compounds is strongly related to dihydrogen bonds. And dihydrogen bonds in alkali-metal amidoboranes improved the dehydrogenation of these materials by producing H2 directly through the combination of Hδ+ and Hδ‑. By the formation of dihydrogen bonds between calcium/ lithium borohydride and GBH, the combination of these materials shows a better hydrogen generation property. Thus, we suggest that the formation of new dihydrogen bonds under high pressure will influence the decomposition of GBH and probably promote the release of H2.
Figure 8. Calculated crystal structures of GBH: (a) at ambient pressure and (b) at 1.0 GPa. The black dashed lines represent the original dihydrogen bonds, and the red dashed lines represent the new generated dihydrogen bonds.
When it comes to a certain pressure, the distance between hydrogens on the guanidinium and that on the borohydride from a different tape become so close that dihydrogen bonds can be formed. The newly generated dihydrogen bonds result in the phase transition of GBH. The proposed mechanism for the phase transition is in accordance with the characteristics of high pressure Raman experiments. Before the phase transformation, the NH stretching modes at 3262 cm−1 exhibited large blue shift, while the other two NH stretching modes did not show an apparent shift. So we assigned the peak at 3262 cm−1 to the vibration mode of NH which did not supply hydrogen to dihydrogen bonds as shown in Figure 1. After the phase transition, all the NH stretching modes displayed a small blue shift, indicating all the hydrogens of guanidinium were involved in dihydrogen bonds. What is more, the Raman vibration modes at 2351 cm−1 (BH stretch region) and 3329 cm−1 (NH stretch region) could be detected at 0.4 GPa. The new emerged bands indicated that dihydrogen bonds have changed, inferring
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CONCLUSIONS In summary, ADXRD techniques and Raman spectroscopy have been utilized to study the changes in dihydrogen-bonded hydrogen storage material GBH at high pressure. A reversible phase transition of GBH was confirmed at approximately 0.4 GPa by both ADXRD and Raman experiments. Through the analysis of first-principles calculation and Raman spectra, we attributed the phase transition to the generation of dihydrogen bonds between different GBH tapes. This study can provide deeper understanding into the nature of dihydrogen bonding. 13418
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And the formation of new dihydrogen bonds may influence the release of H2 and give insight into the improvement of the dehydrogenation for this material.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 86-431-85168882; e-mail:
[email protected] (B.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We are grateful to Dr. Bo Zhou for the synthesis of guanidinium borohydride. This work is supported by the NSFC (No. 91227202) and the Changbai Mountain Scholars Program (No. 2013007). Angle-dispersive XRD measurement was performed at the 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF) which is supported by Chinese Academy of Sciences (Nos. KJCX2-SW-N20, KJCX2-SWN03).
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