Achieving High Dehydrogenation Kinetics and Reversibility of LiBH4

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Cite This: J. Phys. Chem. C 2018, 122, 23336−23344

Achieving High Dehydrogenation Kinetics and Reversibility of LiBH4 by Adding Nanoporous h‑BN to Destabilize LiH Jiuyi Zhu,†,‡ Hui Wang,†,‡ Jiangwen Liu,†,‡ Liuzhang Ouyang,†,‡ and Min Zhu*,†,‡ †

School of Materials Science and Engineering and ‡Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510640, P. R. China

J. Phys. Chem. C 2018.122:23336-23344. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Lithium borohydride (LiBH4) is a potential high-capacity hydrogen storage material but is limited by its high thermal stability and poor reversibility. Nanostructured hexagonal boron nitrides (h-BNs) with thin-layer (TL-BN) and nanoporous (NP-BN) structures have been prepared by treating h-BN in a hydrolysis process of LiBH4 and then mixing with LiBH4 by ball-milling to investigate their catalyzing effect on dehydrogenation and rehydrogenation. Nanostructured h-BN, in particular NP-BN, with an average pore size of 40 nm, significantly promotes the dehydrogenation kinetics, dehydrogenation capacity, and reversibility of LiBH4. For a LiBH4 + NP-BN (mole ratio 1:0.3) composite, the dehydrogenation capacity reaches 13.9 wt % for LiBH4 at 400 °C, which is very close to its theoretical one. The cycling capacity could remain stable at ∼7.6 wt % with rapid kinetics after de/rehydrogenation cycles under 10 MPa of hydrogen at 400 °C. The improved de/hydrogenation performance of LiBH4 depends on the nanoconfinement structure of LiBH4 in NP-BN. After dehydrogenation, a lithium-intercalated h-BN(LixBN) nanocrystal was formed by the reaction of h-BN and LiH. This LixBN nanocrystal confined the dehydrogenation products of the amorphous structure during dehydrogenation. Thus, this nanocrystal-confine-amorphous structure destabilizing LiH and benefiting the rehydrogenation and reversible capacity of the composite. borides23−25 and carbon-based materials,26−28 have been found to function as catalysts without side reactions. For example, nanostructured CoB could facilitate the main dehydrogenation of LiBH4 at around 350 C and retain the hydrogen capacity of LiBH4 at ∼9.6 wt % after four cycles.25 However, the main hydrogen release temperature is still high and the reversibility is still poor for the catalyzed LiBH4. Graphite-like structured hexagonal boron nitride (h-BN), especially in the nanolayer form, shows unique physical properties and good chemical stability, and thus, has been used, for instance, as a lubricant29 and an insulating thermoconductive layer,30 and for ultraviolet-light luminescence.31 The thin h-BN layer can be peeled off easily from bulk h-BN due to the weak van der Waals force between layers. The possibilities of using h-BN as a hydrogen storage material have been explored by first-principles calculations.32−34 Recently, some special h-BNs with a unique nanostructure have been produced to store hydrogen by forming both physical and chemical bonds with hydrogen.35−39 Boron nitride nanotubes with collapsed surfaces could absorb 4.2 wt % hydrogen at room temperature and long-time ball-milled h-BN could store

1. INTRODUCTION As one of the most promising alternative energy carriers, hydrogen has attracted a great deal of attention because of its high energy density and environmentally friendly oxidation product.1−3 However, reversible high-density hydrogen storage is still a great challenge.4 Lithium borohydride (LiBH4) is one of the most potential candidates for on-board solid hydrogen storage due to its high gravimetric and volumetric hydrogen densities, being 18.5 wt % and 121 kg H2/m3, respectively.5−8 However, the high thermal stability and poor reversibility of LiBH4 hamper its practical application. Normally, the main decomposition of LiBH4 starts above 380 °C, and releases only half of the stored hydrogen, about 9 wt %, below 600 °C.9 Moreover, harsh conditions (30 MPa of hydrogen at 600 °C) are necessary for the rehydrogenation of LiBH4.5 To solve these problems, several methods, such as catalysis,9−12 destabilization,13−15 and nanoengineering,16−19 have been attempted in the past years but were not very effective. For example, it is effective to destabilize LiBH4 by forming a LiBH4−MgH2 composite, but the formed compound MgB2 limits the dehydrogenation kinetics.20 As another example, additives such as oxides or halides can catalyze the dehydrogenation of LiBH4, but they react with LiBH4 at high temperatures, and thus, reduce the reversible hydrogen storage capacity of the system severely.21,22 Some additives, such as © 2018 American Chemical Society

Received: July 24, 2018 Revised: September 25, 2018 Published: September 26, 2018 23336

DOI: 10.1021/acs.jpcc.8b07086 J. Phys. Chem. C 2018, 122, 23336−23344

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of the as-prepared h-BN: (a) pristine, (b) TL-BN, and (c, d) NP-BN.

respectively. In this work, the floater and sedimentation layers were carefully removed and washed with ultrapure water 3−5 times by vacuum suction filtration through a microporous membrane with a pore diameter of 0.2 μm. Finally, the powders were freeze-dried using a CHAIST ALPHA1-2LD plus for 12 h. The samples from the floater and sedimentation layers are called nanoporous h-BN (NP-BN) and thin-layer hBN (TL-BN), respectively. 2.1.2. LiBH4 + h-BN Composites. For preparing all LiBH4 + h-BN composites, LiBH4 was pre-milled first on a vibratory ball mill (QM-3C, China) at 1000 rpm for 1 h with a ball-topowder weight ratio of 120:1 under Ar gas protection. Then, the pre-milled LiBH4 and h-BN (pristine, TL-BN, and NPBN) were milled using the vibratory ball mill at 1000 rpm for 2 h with a mole ratio of 1:0.3 and a ball-to-powder weight ratio of 120:1 under Ar gas conditions. The composites are labeled LiBH4 + 0.3 Pr-BN, LiBH4 + 0.3 TL-BN, and LiBH4 + 0.3 NPBN, respectively. 2.2. Structure Characterization. X-ray diffraction (XRD) analysis was performed on a PANalytical-Empyrean diffractometer with Cu Kα radiation. Raman spectra were measured on a Thermo-Fisher Scientific Micro DXR Raman microscope with a 488 nm Ar laser source in a macroscopic configuration. Scanning electron microscopy (SEM) images were obtained with a Nova Nano SEM 430 attached to a Bruker X-ray energy dispersive spectrometer. Transmission electron microscopy (TEM) analyses were conducted on a JEM-2100 transmission electron microscope operating at 200 kV. The samples for TEM observation were dispersed in heptane and then deposited on a perforated carbon film on a Ni grid. 2.3. Kinetics Measurements. The dehydrogenation properties were measured using a temperature-programmed desorption mass spectrometer (TPD−MS). The gaseous species released from the sample during heating at a ramping rate of 4 °C/min was detected under high-purity argon purging with a flow rate of 60 mL/min using a QIC-20 gas analysis system. Dehydrogenation kinetics was measured on a Sieverttype apparatus (PCT-Pro 2000, Setaram) under static vacuum.

2.6 wt % hydrogen; however, desorption took place above 300 °C for both of them.35,37 Oxygen-doped boron nitride nanosheets exhibited good cycling stability up to 15 cycles with ∼5 wt % hydrogen storage capacity, but dehydrogenation was untestable because the stored hydrogen was released simply upon reducing the pressure.38 These preliminary results show the potential of h-BN in hydrogen storage. In our previous works, ball-milled nanocrystal h-BN was composited with LiBH4 as a catalyst to improve the hydrogen storage properties of LiBH4.40−42 It has been found that adding a small amount of h-BN could enhance the dehydrogenation kinetics of LiBH4,41 and the cyclic de/rehydrogenation of LiBH4 would be improved by adding more h-BN.42 It has been revealed that both B and N atoms of h-BN play important roles in this system. The lone pair electrons of the N atom on the surface of h-BN could induce partial destabilization of LiBH4, and newly formed B−H bonds could be observed due to the polarizable B−H bond at the interface between h-BN and LiBH4. Obviously, the surface state of h-BN is the determining factor in enhancing the hydrogen storage properties of LiBH4. Thus, it is expected that h-BN with a stronger surface effect from novel nanostructures can further improve the de/ rehydrogenation properties of LiBH4. In this work, on the basis of the above consideration, different nanostructured hBNs were produced by hydrolysis43 and added into LiBH4 to improve the dehydrogenation and cycling properties of LiBH4.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. 2.1.1. h-BN with Different Nanostructures. Pristine h-BN (Alfa Aesar 99.5%) and LiBH4 (Sigma-Aldrich 95%) were mixed well in an agate mortar in a weight ratio of 1:2. After that, the mixture was heated at 300 °C under 4 MPa of hydrogen for 2 h. After cooling the mixture to room temperature, it was transferred to a glass beaker with ∼100 mL of ultrapure water. The solution was ultrasonically treated for 1 h until the hydrolysis was completed. After 12 h of standing, three separated layers were formed in the solution, including a floater, a suspension, and a sedimentation layer, 23337


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Figure 2. (a) Raman spectra and (b) XRD patterns of the prepared h-BN.

Figure 3. (a) Isothermal dehydrogenation kinetics and (b) TPD−MS curves of LiBH4 + 0.3 h-BN composites.

of different morphologies is very similar. A broad E2g vibrational mode peak near 1370 cm−1 can be observed, which is contributed by the B−N vibrational mode within hBN layers, analogous to the G peak in graphene.44,45 The E2g mode peaks are at 1364.3, 1364.6, and 1364.7 cm−1 for pristine, TL-BN, and NP-BN, respectively. The E2g modes of both TL-BN and NP-BN shift to a little higher frequency compared with that of pristine h-BN. And the full width at halfmaximum of the E2g mode peak decreases to 8.5 and 7.8 cm−1 for TL-BN and NP-BN from 9.2 cm−1 for pristine h-BN, respectively. These results confirm that the size of h-BN is reduced after exfoliation.46,47 In the XRD patterns, all of the samples show full and sharp diffraction peaks of h-BN (Figure 2b). It is worth mentioning that the intensity ratio of (004) to (102) of h-BN increases after the exfoliation. This suggests that the thickness of TL-BN and NP-BN decreased along (100). The formation of TL-BN results from hydrogen generation from the hydrolysis of LiBH4, existing at the interlayers of the curled sheets, which has been described in our previous work.43 With respect to the formation of the porous structure, it should be related to the localized hydrogen generation in the hydrolysis process (Figure S3). When LiBH4 existed on the uneven surface of h-BN (Figure S4), liquid LiBH4 would be deposited on the sags. And then, hydrogen gas was generated at the inner plane of h-BN by the hydrolysis of LiBH4. The hydrogen gas damaged the local areas in the plane of the h-BN and created holes in h-BN. Porous substrates and superior thin flakes would be produced by this type of exfoliation. The porous substrates would float on the solution due to the

Rehydrogenation of the dehydrogenated products was performed at 400 °C under 10 MPa hydrogen for 24 h. The de/rehydrogenation capacity was calculated with respect to the total weight of LiBH4 in the sample. To minimize the H2O/O2 contamination during the rehydrogenation treatment, the hydrogen supply (99.999% purity) was further purified using a hydrogen storage alloy system. All samples were handled in an argon-filled glove box with O2 and H2O levels below 1 ppm.

3. RESULTS 3.1. Structure of Nanostructured h-BN. The morphology of the pristine h-BN and as-prepared nanostructured TLBN and NP-BN is shown by their SEM images (Figure 1). The thickness and the thickness distribution of pristine h-BN and TL-BN were also measured from atomic force microscopy (AFM) analysis (Figure S1). Pristine h-BN has a layered structure with a size of tens of micrometers in diameter and over 50 nm in thickness (Figure 1a). After the exfoliation, TLBN obtained in sedimentation layers shows a smaller diameter than the pristine one, with a thickness of less than 10 nm (Figure 1b). It could be noticed that these TL-BN layers curl along the perimeter, which is in accordance with a previous work.43 A sponge-like structure can be observed for NP-BN obtained in the floater layer (Figure 1c,d). The pore size of NP-BN is mainly ∼40 nm resulted from a pore-diamater distribution after a statistical analysis (Figure S2). The structures of TL-BN and NP-BN were further analyzed by XRD and Raman spectroscopy (Figure 2). For the Raman spectra (Figure 2a), the shape of the peaks for all those h-BNs 23338


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Figure 4. XRD patterns of different LiBH4 + 0.3 h-BN composites (a) after ball-milling and (b) after first dehydrogenation.

Figure 5. FTIR spectra of different LiBH4 + 0.3 h-BN composites (a) after ball-milling and (b) after first dehydrogenation.

above 400 °C upon heating from 100 to 600 °C, corresponding to the fusion and main decomposition of LiBH4. The main hydrogen decomposition peak becomes sharper by adding h-BN, and the temperature of the peak decreases from 450 °C for pre-milled LiBH4 to 435, 435, and 413 °C for LiBH4 added with pristine, TL-BN, and NP-BN, respectively. The sharper peaks at the lower temperature indicate faster dehydrogenation kinetics for all LiBH4 + 0.3 BN samples. Obviously, the LiBH4 + 0.3 NP-BN sample shows the lowest dehydrogenation temperature, highest dehydrogenation capacity, and fastest kinetics. This is owing to the higher surface effect of the porous structure of NP-BN. The structure of the LiBH4 + 0.3 BN composites was analyzed by XRD, Fourier transform infrared (FTIR), and SEM to reveal the mechanism for rapid dehydrogenation kinetics of LiBH4 after adding h-BN. The XRD patterns show the phase change during ball-milling and the first dehydrogenation (Figure 4). Compared with the mixture before ballmilling (Figure S5), the diffraction peaks of both h-BN and LiBH4 vanished after ball-milling. As a characteristic of LiBH4, it would be amorphized easily after an effective ball-milling process.48 In the case of LiBH4 + 0.3 h-BN composites, h-BN may make LiBH4 amorphization more easy as an auxiliary agent for ball-milling. For h-BN, a broadened peak at ∼44° is observed for each ball-milled sample, which is due to the formation of nanocrystalline and even amorphous h-BN.42,49

hydrogen gas present inside them, whereas the superior thin nanoflakes would be suspended in the solution. In this work, TL-BN and NP-BN were collected from the sedimentation and floater layers in the solution for further study. 3.2. Dehydrogenation Properties and the Structure of LiBH4 + 0.3 h-BN Composites. The dehydrogenation properties of LiBH4 + 0.3 h-BN composites were measured by isothermal dehydrogenation kinetics (Figure 3a). Pre-milled LiBH4 shows hindered kinetics of decomposition. Even after 10 h of heating at 400 °C, the sample still released hydrogen and slowly reached a desorption capacity of 10.3 wt %. With the addition of h-BN, all composites show kinetics better than that of pre-milled LiBH4. A rapid hydrogen release starts from 280 °C and the dehydrogenation capacities of LiBH4 are obviously increased, reaching 11.3, 11.9, and 13.9 wt % by adding pristine h-BN, TL-BN, and NP-BN, respectively. In particular, for the LiBH4 + 0.3 NP-BN composite, the capacity is very close to its theoretical one (13.8 wt %, LiBH4 → LiH + B + H2). A similar result has been obtained for the LiBH4 + 3 BN system in which h-BN is the major ingredient.42 This result implies that nanostructured h-BN, in particular NP-BN, is much more effective in catalyzing the dehydrogenation of LiBH4. The dehydrogenation properties were further measured by TPD−MS (Figure 3b). For all samples, the onset of hydrogen release is at ∼280 °C, and the main hydrogen peak is observed 23339


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dehydrogenation capacities are 13.9, 9.6, 7.9, 7.6, and 7.6 wt % for the initial five cycles, respectively. Although an obvious capacity reduction is observed in the initial three cycles in this system, it showed a higher capacity than that obtained previously in Tu’s work by adding 0.3 mol ball-milled h-BN, which was ∼7.0 and ∼6.0 wt %, respectively, for the second and third dehydrogenation.41 Furthermore, the cycling capacity could be stabilized for this system. And, more remarkably, the rapid kinetics and a similar dehydrogenation trend were retained when the LiBH4 + 0.3 NP-BN composite underwent heating at temperatures up to 400 °C during the cycle. The change of the structure for LiBH4 + 0.3 NP-BN composite after de/rehydrogenation cycles was analyzed by XRD and FTIR (Figure 7). For both rehydrogenation and dehydrogenation samples, stronger intensity of the peaks was observed from the XRD pattern of the rehydrogenated sample than that of the ball-milled one. This is due to the grain growth of the LiH and B during the de/rehydrogenation cycle at high temperatures. The growth of LiH during cycling is also observed in the TEM image shown in Figure S7. Meanwhile, the existence of LiH and B illustrated that a fraction of the dehydrogenation product could not reversibly transform back into LiBH4. It is noted that the diffraction peaks of both h-BN and LiBH4 are still absent even after cycling at a high temperature (400 °C) for a long time (24 h), which means the recycled LiBH4 still retains its amorphous structure. For the FTIR spectra (Figure 7b), the rehydrogenation sample shows strong and sharp B−H bonds from [BH4]− at 2386, 2291, 2223, and 1124 cm−1. The mentioned broad feature corresponding to Li2B12H12 at ∼2450 cm−1 is still observed during the cycle because a fraction of the products could not be transformed back into LiBH4. Moreover, several characteristic peaks of B−H bonds at 1000−1100 cm−1 are detected, which are similar to the ones of B−N−H compounds (PAB oligomers), which could decompose at around 150 °C.52,53 The exothermicity of hydrogen release from B−N−H compounds could be decreased by the interface of BN support.54 Thus, these novel B−H bonds from B−N−H compounds may retain the rapid kinetics of the dehydrogenation of the LiBH4 + 0.3 NP−BN composite during the cycle in the low-temperature area.54 The agglomeration of grown LiH and the stable Li2B12H12 are the important reasons for the decrease in hydrogen capacity during the cycle.

Uniformly mixed h-BN and LiBH4 composites could also be observed from the SEM images of the ball-milled samples (Figure S6). After the ball-milling process, the area of contact between h-BN and LiBH4 increased and would be the key factor for the rapid kinetics of the dehydrogenation of these systems. After heating at 400 °C for 10 h, LiH could be found in all samples (Figure 4b). The formation and the grain growth of LiH is due to the decomposition of LiBH4 (LiBH4 → LiH + B + H2). The reaction of LiH and water in air may result in the existence of a small amount of LiOH. FTIR spectra of LiBH4 + 0.3 h-BN composites show the formation of new bonds after ball-milling and the first dehydrogenation (Figure 5). For all composites after ballmilling, strong bonds at 810 and 1385 cm−1 are observed corresponding to the out-of-plane B−N−B bending vibration and in-plane B−N bond stretching vibration from h-BN, respectively. The characteristic B−H vibrations from [BH4]− at 2386, 2291, 2223, and 1124 cm−1 are also detected. These B− N and B−H bonds indicated that LiBH4 and h-BN were uniformly mixed after the ball-milling process. After the first dehydrogenation, strong B−N bonds from h-BN and weakened B−H bonds from [BH4]− were observed. Meanwhile, a broad feature at ∼2450 cm −1 is observed corresponding to the B−H stretching modes of Li2B12H12, which is one of the intermediate products from LiBH4.50,51 These results indicated that LiBH4 decomposition is still incomplete with the catalyzation of h-BN. As mentioned above, the dehydrogenation capacity could nearly reach its theoretical value for the LiBH4 + 0.3 NP-BN composite. And the results of XRD also indicate the reaction of LiBH4 → LiH + B + H2. However, the B−H bonds from [BH4]− and Li2B12H12 are observed from FTIR spectra after the first dehydrogenation. These results imply that LiH may contribute part of hydrogen released at high temperature. 3.3. Cycling Properties and the Structure of the LiBH4 + 0.3 NP-BN Composite. The cycling isothermal dehydrogenation properties of LiBH4 + 0.3 NP-BN have been measured after rehydrogenation at 400 °C and under 10 MPa of hydrogen for 24 h, which are named second to fifth dehydrogenation (Figure 6). The hydrogen storage capacity of the system decreases with increasing number of cycles and gradually becomes stable in the fifth dehydrogenation. The

4. DISCUSSION A similar dehydrogenation trend for LiBH4 + 0.3 NP-BN during the cycling process could be explained by the TPD−MS curve (Figure S8). The result is in good accordance with the isothermal dehydrogenation curve (Figure 6). The first peak, at around 140 °C, should correspond to the first step of dehydrogenation with about 0.3 wt % hydrogen release. This decomposition at very low temperature may be due to the breaking of the B−H bond formed on the surface of h-BN, as just mentioned before. The second peak is contributed by the decomposition of LiBH4 with a low onset temperature of around 300 °C, corresponding to the rapid dehydrogenation from the melting point of LiBH4 to 400 °C with the release of about 2.0 wt % hydrogen. A high-temperature gas release, corresponding to the third peak, appears when the temperature increases above 400 °C. This part of dehydrogenation should be contributed by the decomposition of LiH catalyzed by h-

Figure 6. Isothermal dehydrogenation curves of LiBH4 + 0.3 NP-BN composite after rehydrogenation at 400 °C and under 10 MPa of hydrogen for 24 h. 23340


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Figure 7. (a) XRD patterns and (b) FTIR spectra of LiBH4 + 0.3 NP-BN before and after the fifth dehydrogenation.

Figure 8. SEM images of the LiBH4 + 0.3 NP-BN composite (a) before the fifth decomposition, (b) after the fifth decomposition and TEM images and selected-area diffraction of the LiBH4 + 0.3 NP-BN composite (c, f) after ball-milling, (d, g) before the fifth decomposition and (e, h) after the fifth decomposition.

BN, whereas LiH without the catalyst decomposes above 600 °C. However, the kinetics of the third part is very slow in the isothermal dehydrogenation curve because the heating temperature is limited to 400 °C. To prove the catalytic effect of h-BN on the decomposition of LiH, an additional experiment has been done by synthesizing a LiH + h-BN system (Supporting Information, Figure S9).

The morphology of the LiBH4 + NP-BN mixture during de/ rehydrogenation was observed by SEM (Figure 8) to explain the improved reversible hydrogen capacity. After the ballmilling process (Figure S10), well-mixed LiBH4 and h-BN could be observed undergoing stratification. A smooth surface of the mixture was shown after the first dehydrogenation due to the fusion of LiBH4 at a high temperature (400 °C). The morphology of the sample after four cycles is shown in Figure 23341



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8a,b. For the rehydrogenated sample, two kinds of structure could be observed. The rectangular area should be contributed by the small bulk of grown LiH, B or Li2B12H12 during the cycle and the smooth area should be contributed by the recovered LiBH4. For the dehydrogenated sample (Figure 8b), the nanoscale homogeneous particles of the decomposition product could be observed on the surface of the mixture. These small particles could re-form LiBH4 reversibly during the rehydrogenation process. The structure of these nanoparticles is further analyzed by TEM observation (Figure 8c− h). For the ball-milled sample (Figure 8c,f), the structure is mainly amorphous, which is in agreement with the results of XRD patterns (Figure 4a). However, a small fraction of nanocrystalline h-BN with an interplanar distance of 3.3 Å could also be found, which is in accordance with pristine h-BN. This indicates that LiBH4 and a major part of h-BN are amorphized. It should be noted that the amorphous structure could be maintained for the sample in the hydrogenated state even after four cycles (Figure 8d,g). This is quite different from the situation without h-BN addition in which LiBH4 is generally in the crystalline state in the hydrogenated sample. For the dehydrogenated sample after the cycling process (Figure 8e,h), an amorphous structure wrapped with nanocrystals could be observed. The interplanar distance of this nanocrystal is slightly larger than that of the ball-milled sample of 3.6 Å. This should be because the Li-intercalation expanded the interplanar distance of h-BN, i.e., by forming LixBN.55−57 The amorphous structure should be a mixture of dehydrogenation products including LiH, Li2B12H12, and B. Thus, it is a structure of amorphous dehydrogenation products confined by LixBN. This structure, on the one hand, could provide more dehydrogenation capacity by destabilizing LiH with the reaction of xLiH + BN → LixBN + x/2H2; on the other hand, this structure improved the cycling dehydrogenation capacity of the system because the LixBN nanocrystal confined the dehydrogenation products effectively, and a steady capacity of 7.6 wt % was obtained after five cycles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07086.

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AFM images of pristine h-BN and TL-BN, pore distribution of NP-BN, schematic of in-plane exfoliation by LiBH4, surface topography of pristine h-BN, XRD patterns of the mixture before ball-milling, SEM images of the mixture after ball-milling, TEM images and selected-area diffraction of rehydrogenated LiBH4 + 0.3 NP-BN, TPD−MS of LiBH4 + 0.3 NP-BN 5th dehydrogenation, preparation and properties of the LiH + h-BN mixture, SEM images of the LiBH4 + 0.3 NP-BN composite (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liuzhang Ouyang: 0000-0003-2392-2801 Min Zhu: 0000-0001-5018-2525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51621001), National Natural Science Foundation of China (Nos. U1601212, 51431001), and by the Project Supported by the Natural Science Foundation of Guangdong Province of China (2016A030312011). Support by the Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014) is also acknowledged.

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5. CONCLUSIONS Two types of nanostructured h-BN have been prepared by exfoliation, assisted by the hydrolysis of LiBH4, and thin-layer h-BN (TL-BN) of around 10 nm thickness and porous structured h-BN (NP-BN) with a pore size of around 40 nm were obtained. These nanostructured h-BNs were ball-milled with LiBH4 successively to catalyze their decomposition. In particular, the NP-BN added sample showed the best kinetics and the highest dehydrogenation capacity of 13.9 wt %, which was close to the theoretical one. Moreover, this LiBH4 + 0.3 NP-BN composite showed rather good cycling properties with a stable capacity of 7.6 wt % after five dehydrogenation/ rehydrogenation cycles. The excellent dehydrogenation kinetics of LiBH4 + 0.3 NP-BN can also be maintained after the de/rehydrogenation cycles up to 400 °C. The LiBH4 + 0.3 NP-BN composite is mainly amorphous in structure in the hydrogenated state during the cycling, which may be a reason for the fast dehydrogenation kinetics. For the dehydrogenated sample, an amorphous structure, which is a dehydrogenation product of LiBH4, wrapped with nanocrystalline LixBN was observed. This structure could destabilize LiH and confine the dehydrogenation products to promote the rehydrogenation process and the reversible hydrogen capacity of the system during the cycling.

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Achieving High Dehydrogenation Kinetics and Reversibility of LiBH4

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