Insight to the Thermal Decomposition and Hydrogen Desorption

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Insight to the thermal decomposition and hydrogen desorption behaviors of NaNH2-NaBH4 hydrogen storage composite Ziwei Pei, Ying Bai, Yue Wang, Feng Wu, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10043 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

Insight to the thermal decomposition and hydrogen desorption behaviors of NaNH2-NaBH4 hydrogen storage composite Ziwei Pei,† Ying Bai,*,† Yue Wang,† Feng Wu, †,‡ Chuan Wu*,†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of

Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR

China

KEYWORDS: hydrogen storage, NaNH2-NaBH4, ball-milling, dehydrogenation, thermal decomposition ABSTRACT: The light-weight compound material NaNH2-NaBH4 is regarded as a promising hydrogen storage composite due to the high hydrogen density. Mechanical ball-milling was employed to synthesize the composite NaNH2-NaBH4 (2/1 molar ratio), and the samples were investigated utilizing Thermogravimetric-differential thermal analysis-mass spectroscopy (TG-DTA-MS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses. The full-spectrum test (range of the ratio of mass to charge: 0-200) shows that the released gaseous species contain H2, NH3, B2H6 and N2 in the heating process from room temperature to 400 °C, and possibly the impurity gas B6H12 also exists. The TG/DTA analyses show that the composite NaNH2-NaBH4 (2/1 molar ratio) is conductive to generate hydrogen that the dehydrogenation process can be finished before 400 °C. Moreover, the thermal decomposition process from 200 °C to 400 °C involves two steps dehydrogenation reactions: (1) Na3(NH2)2BH4 hydride decomposes into Na3BN2 and H2 (200-350 °C); (2) remaining Na3(NH2)2BH4 reacts with NaBH4 and Na3BN2, generating Na, BN, NH3, N2 and H2 (350-400 °C). The better mechanism understanding of the thermal decomposition pathway lays a foundation for tailoring the hydrogen storage performance of the composite complex hydrides system. 1

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INTRODUCTION Fossil fuel shortage and increasingly wicked ecological actuality have become potential threats to human livings and social development.1 Hydrogen, as a promising energy alternative, is characterized as its high energy of 142 MJ kg-1 and environmental friendly nature;2 moreover, hydrogen, as a secondary energy carrier, has to be stored efficiently in security and economic applications. Currently, extensive effort is being devoted to exploring hydrogen storage, which is a key technology for hydrogen fuel cells used in mobile transport,3 and the solid-state hydrogen storage is considered to be one of the main ways of the future hydrogen storage due to its high security and convenience for application. In recent studies, the solid-state hydrogen storage materials can be mainly divided into three aspects:4 1) the molecular high surface area materials which can absorb hydrogen;5,6 2) the metal alloy materials;7,8 and 3) the light-weight complex hydrides, such as the alanates (AlH4-),9-12 borohydrides

(BH4-),13-16 and

amides/imides (NH2-/NH2-)

systems.17-19

U.S.

Department of Energy (DOE) set the targets for fuel cells for automotive, stating that the gravimetric capacity need reach more than 5.5 wt% in 2025.20 Metal borohydrides are one group of compounds exhibiting high weight capacities and therefore considered as candidates to meet the target. Sodium borohydride (NaBH4), which contains 10.8 wt% hydrogen and has a reasonable price, is very attractive as a promising material for solid-state hydrogen storage apart from the hydrolysis.21,22 However, the spatial configuration of [BH4] group is tetrahedron structure, which is so stable that solid-state NaBH4 desorbs hydrogen at relatively elevated temperatures. Recently, lots of studies of improving the kinetics through catalysis23-25 and nanoconfinement26-29 have been done. Besides, with lower stability, the metal-B-N-H system has become one of the interests in the field of hydrogen storage material.30-37 It can form a distinctive interaction, i.e., N–Hδ+/Hδ-–B, which is defined as the‘‘dihydrogen bond’’.38,39 In 2005, the new quaternary hydride Li3BN2H8, composed of two complex hydrides LiNH2 and LiBH4 in molar ratio 2:1, was introduced.40 The composite of LiNH2/LiBH4 (2/1) has a high theoretical hydrogen capacity of 11.9 wt%. The corresponding NaNH2-NaBH4 system 2


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promises to be more appropriate for the development of new hydrogen storage materials, with the price one tens of that of LiBH4.41,42 Furthermore, the electronegativity of metal Li and Na are almost the same, 1.0 and 0.9, respectively. We did a lot of investigations on the dehydrogenation performance of NaNH2-NaBH4 (2/1 molar ratio), such as the desorption property of pristine,43 and Co-B,44 Ni-Co-B,45,46 and Mg-Co-B doped NaNH2-NaBH4 (2/1 molar ratio) hydrogen storage materials.47 The activation energy for the Co-B doped NaNH2-NaBH4 (2/1 molar ratio) is only 70 KJ/mol, 43.9% of that of the pristine NaNH2-NaBH4 (2/1 molar ratio). In addition, it achieves a 76.4 KJ/mol low activation energy simply by liquid phase ball-milling.48 However, the gas products and reaction mechanism during the hydrogen desorption are still unclear and need further investigation. Similar investigations of NaNH2-Ca(BH4)2 and NaNH2-Mg(BH4)2 have been done.49 In this paper, NaNH2-NaBH4 (2/1 molar ratio) is synthesized by ball-milling. The hydrogen generation performance is elucidated in detail utilizing the TG-DTA-MS test. Moreover, the decomposition products of NaNH2-NaBH4 (2/1 molar ratio) and its thermal decomposition pathway have been investigated. Refined decomposition process will help analyze the reaction mechanism of different stages. So that special measures can be taken to improve the dehydrogenation performance periodically, especially in the selection of catalyst and inhibition of gaseous impurities. Further study will be conducted on NaNH2-NaBH4 as a reversible hydrogen storage material.

EXPERIMENTAL SECTION Syntheses. Commercially available NaBH4 (Tianjin Hainachuan science and technology development Co., Ltd., 98% assay) and NaNH2 (Sigma-Aldrich，98% assay) were used. The composite NaNH2-NaBH4 (2/1 molar ratio) samples were synthesized by the ball-milling technique. In the argon-filled glove box (MBRAUN), NaNH2 and NaBH4 (2/1 molar ratio) were weighed and put into a ball-milling jar. The weight ratio of the steel balls to the raw materials was 20:1. After sealing the ball-milling jar, it was thrown into a planetary ball mill (QM-3SP2), as shown in Figure 1. The rotating speed was 300 rpm 3



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and the operation in reverse was carried out. After the ball-milling, the jar was taken down and put into the glove box, and then the synthesized sample was taken out. To avoid the reaction between the materials and the moisture and O2, all the operations were in a high purity argon (H2O≤1 ppm, O2≤1 ppm) atmosphere. TG-DTA-MS analysis. The thermogravimetric analysis-differential thermal analysis-mass spectroscopy (TG-DTA-MS) test was conducted in a Diamond TG-DTA/DSC simultaneous thermal analyzer (PerkinElmer) and a ThermostarTM gas analysis mass spectrometer (Pfeiffer), as shown in Figure 1. The injection pressure was 100 mbar and the high purity argon (99.999%) gas flow was 2 mL/min. In the glove box, about 4 mg sample was put in a corundum crucible, and then quickly shifted to the TG-DTA-MS instrument. The samples were measured from room temperature to 450 °C with a heating rate of 10 °C/min.

Figure 1. Schematic illustration for the characterization and preparation of the composite of NaNH2-NaBH4 (2/1 molar ratio).

X-ray diffraction analysis (XRD). X-ray diffraction (XRD) examinations were conducted on a Rigaku DMAX2400 X-ray diffractometer adopting Cu-Ka radiation with the tube voltage of 40 kV, step of 0.02° and tube current of 20 mA. The scanning range and rate was from 10° to 80° and 8°/min respectively. In order to characterize the decomposition products of NaNH2-NaBH4 (2/1 molar ratio), appropriate amount of the samples was heated for 30 min at 150, 200, 330, 350 and 400 °C respectively in the tubular furnace, where the protective gas was high purity argon (99.999%) and the heating rate was 5 °C/min. The original composite and samples in different conditions of thermal treatment were measured and analyzed by XRD. 4


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FTIR spectra analysis. The FTIR spectra instrument adopted in this study was a FTS-60V Double vacuum Fourier infrared spectrometer (Bio-Rad, America) with the spectral resolution of 0.1 cm-1, the wavenumber precision of 0.01 cm-1, and the signal-to-noise ratio (S/N) of 6000:1. The scanning wavenumber range was the mid-infrared range from 4000 to 400 cm-1. The samples for FTIR tests were contained in KBr powder. The mixture was pressed into a sample disk in the glove box.

RESULTS AND DISCUSSION Dehydrogenation performance of the NaNH2-NaBH4 (2/1 molar ratio) composite. In order to further confirm the compositions of the released gases in the thermal decomposition process, both the full spectrum and single channel tests were conducted. The full spectrum pattern of the composite NaNH2-NaBH4 (2/1 molar ratio) is shown in Figure 2a, and the range of the ratio of mass to charge is from 0 to 200. The horizontal, vertical and the third axis represent the ratio of mass to charge, relative intensity, and time (1cycle=4s) respectively. The full spectrum pattern (Figure 2a) shows that there is no gas production in the range of the ratio from 40 to 200 except for ca. 77. Therefore, during the thermal decomposition process, it is likely that the cyclic borane (B6H12) is generated, and the amount of the generated B6H12 is small. The concluded reaction is expressed as the following. 6NaBH4 → B6H12+6NaH+3H2

(1)

In order to further investigate the released gaseous species, the screenshots of the full spectrum with the ratio range from 20 to 30 and 0 to 20 are evaluated, as shown in Figure 2b and Figure 2c, respectively. The existence of the obvious peak from 27.7 to 28 amu (unified atomic mass unit), Figure 2b, indicates the evolution of diborane boroethane (B2H6). The emission of toxic B2H6 during the desorption of borohydride has also been found by Liu et al.50 This phenomenon manifests that the synthesis of NaNH2-NaBH4 (2/1) composite through ball-milling is not sufficient, and there still exists unreacted NaBH4. There are notable fluctuations corresponding to the mass of 14, 16, 17 and 18 amu, as shown in Figure 2c. In the investigation of the NaNH2-NaBH4 composite, Somer et al.42 found that the major component of the 5



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released gases is H2 (m/e=2) (89.3 wt%), accompanying with NH3 (m/e=17) (6.5 wt%). In this study, according to the characteristic peaks of the MS analysis, except B2H6, B6H12 and the main gas H2 (m/e=2), the remaining released gaseous species contain NH2- (m/e=16), NH4+ (m/e=18) and N (m/e=14). It is known that NH3 will ionize weakly under the condition of electric field; the reaction is as follows, 2NH3 → NH2-+NH4+

(2)

The thermal decomposition process of the composite NaNH2-NaBH4 (2/1 molar ratio) generates ammonia, which explains the fluctuation on the ratio of 17. Although the atom N could not exist in gaseous formation, there is N2 with corresponding mass of 28, as shown in Figure 2b. In other words, the atom of N may exist in the formation of N2. In summary, the gaseous species contain H2, NH3, B2H6, N2 and B6H12.

6


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Figure 2. The MS full-spectrum curves of the composite of NaNH2-NaBH4 (2/1 molar ratio) ball-milled for 16h; mass to charge ratio (a) from 0 to 200, (b) from 20 to 30, (c) from 0 to 20.

From the thermogravimetric (TG) curve in Figure 3a, the thermal decomposition process can be divided into two sections in the temperature range from 50 °C to 400 °C. In the first decomposition section from 50 °C to 200 °C, the weight loss is approximately 0.22 wt%. The onset temperature of the significant weight loss is at about 240 °C, and the total weight loss is 6.44 wt% from 240 °C to 400 °C. The mass spectrum test (m/e=17, NH3) shows that ammonia is the primary decomposition product in the first section, and the peak occurs at 134.6 °C. The trace of ammonia at 354.3 °C is also detected. Chater et al.41 found that the desorption of ammonia from NaNH2 starts at around 280 °C. In this study, the ammonia releases at around 130 °C, which is lower than NaNH2. According to the TG-MS analysis, the decomposition of the composite NaNH2-NaBH4 (2/1 molar ratio) starts with the decomposition of amino sodium. Synchronous MS curve (m/e=2, H2) has four peaks which are at 133.5, 266.1, 329.9 and 352.6 °C, respectively. The initial dehydrogenation temperature is as low as 133.5 °C, and the release of hydrogen indicates that the composite is conducive to the generation of hydrogen and restrains the generation of ammonia. The next three peaks are accompanied with a large amount loss of weight corresponding to the major hydrogen desorption steps. Urgnani et al.51 demonstrated that the decomposition reaction of NaBH4 can take place in the solid state when the hydrogen partial pressure is less than 2.5×104 Pa, and NaBH4 can release a lot of hydrogen from 450 °C to 475 °C. The composite NaNH2-NaBH4 (2/1 molar ratio), as a hydrogen storage material, has a lower decomposition temperature. The main stage of the hydrogen generation reaction is before 400 °C, which means the thermal stability of the composite is efficiently reduced. There are three evident peaks in the DTA curve, as shown in Figure 3b. The first endothermic peak is at 149.1 °C and the extrapolated onset temperature is at 138.1 °C, which represents the phase transition process, and the extrapolated onset temperature 7



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is at 138.1 °C. In addition, the synchronous MS curve shows a small hydrogen peak during the phase transition process. According to the MS curve of hydrogen, there are two significant peaks at the main stage of hydrogen generation, and the synchronous DTA curve has two exothermic peaks at this stage, 332.6 °C and 351.1 °C, respectively. Therefore, it is clear that the process of hydrogen generation is exothermic. Furthermore, the two significant hydrogen generation peaks of the MS curve prove that the dehydrogenation of the compound NaNH2-NaBH4 (2/1 molar ratio) involves two steps.

Figure 3. (a) TG-MS and (b) DTA-MS curves of the composite of NaNH2-NaBH4 (2/1 molar ratio) ball-milled for 16h.

Composition and structural evolution of the NaNH2-NaBH4 (2/1 molar ratio) composite. In order to further investigate the chemical reaction mechanism of the thermal decomposition process, the composite NaNH2-NaBH4 (2/1 molar ratio) were heated up to different temperatures based on the above analysis, and FTIR and XRD measurements were carried out. As shown in Figure 3, in the first decomposition section from 50 °C to 200 °C, ammonia is the primary decomposition product in the first section, and the peak occurs at 134.6 °C. From 200 °C to 400 °C, it is strongly discernible that the dehydrogenation of the compound NaNH2-NaBH4 (2/1 molar ratio) involves two steps, and MS curve (m/e=2, H2) has two peaks which are at 329.9 and 352.6 °C. Therefore, the temperature nodes of 150, 200, 330, 350, and 400 °C are chosen. The XRD patterns of the composite NaNH2-NaBH4 (2/1 molar ratio) untreated and heated at different temperatures (150, 200, 330, 350, and 400 °C) are 8


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shown in Figure 4. From the XRD patterns, the phases of NaNH2, NaBH4 and the ball-milled product Na3(NH2)2BH4 with the diffraction peaks at 2θ=12.56, 12.86, 37.48, 38.10, 38.44 and 39.20 can be detected,43 which implies that there is a reaction between the pristine materials in the ball-milling process. After heating to 150 °C, the peak of Na3(NH2)2BH4 at 2θ=12.56 becomes the dominate one as shown in Figure 4b. The peaks at 2θ=48.92, 51.24, 55.44, 59.91, 66.21, 67.90 and 75.40, matching the characteristic peaks of NaBH4, are extremely weak and even disappear when compared with the untreated sample. In addition, the generation of BN and NaH at 2θ=27.26 and 36.96, respectively, demonstrate the occurrence of decomposition. Moreover, associated with the MS curves of H2 (m/e=2) and NH3 (m/e=17) in this stage, the peaks of hydrogen and ammonia at 133.5 °C and 134.6 °C, also prove it. Therefore, the above analysis demonstrates a further generation of Na3(NH2)2BH4 and a preliminary decomposition of the existing Na3(NH2)2BH4. During the heating process, the reactions of this step, producing NaH and BN, and releasing hydrogen and ammonia, can be expressed as the follows, reactions (3) and (4). 2NaNH2+NaBH4 → Na3(NH2)2BH4

(3)

Na3(NH2)2BH4 → 3NaH+BN+NH3+H2

(4)

The peak of NaBH4 at 2θ=28.88 increases after 150 °C, and then decreases as the temperature is increased to 200 °C, as shown in Figure 4b. Just as the reaction (1), the decomposition of NaBH4 may react in this heating treatment, yielding B6H12, NaH and releasing hydrogen. Furthermore, the Na3(NH2)2BH4 exhibits a stronger intensity diffraction peak at 2θ=38.64, which means that a sequential heating procedure can further facilitate the formation of Na3(NH2)2BH4. The possible reactions of this step are as the reactions (3) and (1). After being heated to 330 °C, the new phase of Na3BN2 is observed along with weaker peaks of Na3(NH2)2BH4 at 2θ=12.56 and 38.64, respectively, as shown in Figure 4b and Figure 4c. According to the above investigation, the temperature range of the main weightlessness is from 240 °C to 400 °C, where a significant hydrogen releasing peak is found at 332.6 °C. It shows that the main dehydrogenation reaction 9



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of the composite NaNH2-NaBH4 (2/1 molar ratio) tends to take place at higher temperatures. The reaction of this step is as follows, Na3(NH2)2BH4 → Na3BN2+4H2

(5)

After the temperature is increased to 350 °C, the XRD pattern is almost the same as the XRD pattern of 330 °C. However, after being heated to 400 °C, the peaks at 2θ=29.30 and 53.76 are consistent with the strong characteristic diffractions of metal Na. The peak at 2θ=41.36 is accordance with the BN characteristic diffraction. The peaks at 2θ=32.98, 33.50, 35.50, 36.10, 38.38, 39.38, 41.36, 53.76, 58.26 and 72.72 are corresponding to the characteristic diffractions of Na3BN2, and the weak peak at 2θ=28.00 is of the characteristic diffraction of NaNH2. Therefore, the final decomposition products of the composite are Na3BN2, metal Na, small amount of BN and partial undecomposed NaNH2. Meanwhile, the typical diffraction peaks of NaBH4 and NaH disappear, which means the complete decompositions of NaBH4 and NaH take place at this temperature. As mentioned above, a significant hydrogen peak at 352.6 °C and an additional ammonia release at 354.3 °C are traced, as shown in Figure 3. In addition, by the full spectrum test, very little nitrogen is traced during the decomposition as well. The reactions can be described as follows, Na3(NH2)2BH4+NaBH4+2Na3BN2 → 10Na+4BN+(9/2)H2+NH3+(1/2)N2

(6)

NaH → Na+(1/2)H2

(7)

Figure 4d shows the FTIR spectra of the composite after ball-milling and the decomposed products at different temperatures (150, 200, 330, 350 and 400 °C), which provides evidences for the decomposition pathway. The as-milled sample exhibits the characteristic bands of B-H stretching vibrations of 2226, 2293 and 2293 cm-1, a B-H bending vibration of 1129 cm-1, and a N−H stretching vibration of 1444 cm-1. After being heated to 150 °C and 200 °C, no obvious difference exists for the two samples. Although the diffraction peaks of NaBH4 in the XRD pattern are extremely weak or even disappear while comparing with the as-milled sample, the bands of 1129, 1444, 2226, 2293 and 2293 cm-1 are not shifted, which confirms that a sequential heating procedure can further facilitate the formation of Na3(NH2)2BH4 with typical B−H and N−H vibrations. With the increase of temperature, these 10


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characteristic bands remain but become weak, indicating the consumption of the BH4/NH2 groups. The occurrence of decomposition is also observed in the MS experiment corresponding to the gases release, and the XRD experiment that the new phase of Na3BN2 is observed along with weaker peaks of Na3(NH2)2BH4. After being heated to 400 °C, for the dehydrogenated sample, no characteristic bands of B-H stretching vibrations are observed. It indicates the breaking of B-H bonds and complete decomposition of BH4 groups in the thermal decomposition process. However, there is still a N−H stretching vibration of 1444 cm-1, which means that there is still partial undecomposed NaNH2. It is consistent with the characteristic diffraction of NaNH2 in the XRD pattern. The intensities of characteristic peaks detected gradually become weaker as the temperature increases, showing that the composite of NaNH2-NaBH4 (2/1 molar ratio) releases the most hydrogen after 200 °C. Combining with the above TG-MS curves, if we assumed that the total weight loss in the first decomposition section from 50 °C to 200 °C was caused by ammonia and the impurity gas was almost the ammonia, the weight radio of ammonia from 240 °C to 400 °C would be less than 0.22 wt%, and therefore the purity qualitatively of hydrogen in this stage would be more than 96.5% (1-0.22 wt%/6.44 wt%).

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Figure 4. XRD patterns of the samples after heated at different temperatures (unheated, 150 °C, 200 °C, 330 °C, 350 °C and 400 °C); 2θ (a) from 10 to 80, (b) from 10 to 35, (c) from 35 to 50; (d) FTIR spectra of the samples after heated at different temperatures (unheated, 150 °C, 200 °C, 330 °C, 350 °C and 400 °C).

Reaction mechanism of the NaNH2-NaBH4

(2/1

molar

thermal decomposition process of the

ratio) composite.

In

summary,

the

thermal

decomposition process of the composite of NaNH2-NaBH4 (2/1 molar ratio) ball-milled for 16 h can be described as several reactions, as shown in Figure 5a. In the process of ball-milling, partial NaNH2 reacts with NaBH4 to form the compound Na3(NH2)2BH4. In the decomposition stage from 50 °C to 200 °C, a sequential heating procedure further facilitates the formation of Na3(NH2)2BH4, just as the reaction (3). Meanwhile, the decompositions of a small quantity of Na3(NH2)2BH4 and NaBH4 happen, with the total weight loss about 0.22 wt%. Then, in the decomposition stage from 200 °C to 350 °C, Na3(NH2)2BH4 hydride decomposes into Na3BN2 and H2. If 12


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the temperature is further increased to 400 °C, the remaining Na3(NH2)2BH4 reacts with NaBH4 and Na3BN2, generating Na, BN, NH3, N2 and H2, meanwhile, NaH further decomposes into Na and H2. Moreover, the principal reaction process can be divided into three stages in the temperature range from 50 °C to 400 °C, as shown in Figure 5b, and R1, R2 and R3 represent equation (3), equation (5) and equation (6) respectively. 1) NaNH2 continuously reacts with NaBH4 to form compound Na3(NH2)2BH4, from 50 °C to 200 °C (R1, Figure 5). 2) Partial Na3(NH2)2BH4 decomposes into Na3BN2 and H2 (R2, Figure 5), from 200 °C to 350 °C. 3) The remaining Na3(NH2)2BH4 reacts with NaBH4 and Na3BN2, generating Na, BN, NH3, N2 and H2 (R3, Figure 5). Therefore, there are two steps dehydrogenation reactions of Na3(NH2)2BH4 hydride from 200 °C to 400 °C, corresponding to the two significant hydrogen generation peaks of the MS curve respectively.

13



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Figure 5. Schematic of (a) the thermal decomposition process of the composite of NaNH2-NaBH4 (2/1 molar ratio) ball-milled for 16 h; (b) the primary reactions, composed by R1 (before 200 °C including the ball-milling process), R2 (from 200 °C to 350 °C) and R3 (from 350 °C to 400 °C).

CONCLUSION The generated gaseous composition and the reaction mechanism of the thermal decomposition of the composite NaNH2-NaBH4 (2/1 molar ratio) are investigated, by 14


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the MS, TG-DTA, XRD and FTIR analyses. NaNH2-NaBH4 (2/1 molar ratio), composed of two complex hydrides NaBH4 and NaNH2, is synthesized via ball-milling. The MS analysis shows that the by-product gaseous species contain NH3, B2H6, N2 and B6H12, and the ammonia is the main impurity. The TG analysis shows that the total weight loss in the stage (240-400 °C) is 6.44 wt%. Assumed that the impurity gas was almost the ammonia, the purity qualitatively of hydrogen in this stage would be more than 96.5%. The DTA analysis illustrates that the phase change starts at 138.1 °C and the two exothermic peaks corresponding to the two hydrogen production peaks are at 332.6 °C and 351.1 °C, respectively. The dehydrogenation mainly occurred from 200 °C to 400 °C. The XRD and FTIR patterns indicate that: firstly, a sequential heating procedure can further facilitate the formation of Na3(NH2)2BH4 with temperature lower than 200 °C, and there is little H2 releasing; secondly, at higher temperatures (200-400 °C), a large amount of hydrogen can be generated through the decomposition of Na3(NH2)2BH4. From 200 °C to 350 °C, Na3(NH2)2BH4 decomposes into Na3BN2 and H2. When the temperature is increased to 400 °C, Na3(NH2)2BH4 decomposes into Na, BN, NH3, N2 and H2. The formation of Na3(NH2)2BH4 and its dehydrogenation are favorable responses, which can be promoted through choosing targeted catalyst. The decomposition reaction (Na3(NH2)2BH4 → Na3BN2+4H2) reacted from 200 °C to 350 °C is favorite that there are no gaseous impurities. Better understanding the decomposition process provides reliable reference for the design of the reaction path. Meanwhile, learning the compositions of the released gases helps specifically inhibit impurity gases production, which will facilitate the practicability of composite complex hydrides system.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Bai); [email protected] (C. Wu).

NOTES The authors declare no competing financial interest. 15



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ACKNOWLEDGMENTS The present work is supported by National Natural Science Foundation of China (Grant No. 21476027), and the National Basic Research Program of China (Grant No. 2015CB251100).

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Insight to the Thermal Decomposition and Hydrogen Desorption

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