Significantly improved de/rehydrogenation properties of lithium borohydride modified with hexagonal boron nitride

Abstract

The remarkable hydrogen de/absorption properties of lithium borohydride are achieved by mechanically milling LiBH₄ with hexagonal boron nitride (h-BN). It is found that the dehydrogenation properties of LiBH₄ are improved with increasing the amount of h-BN. The 30 mol% h-BN doped LiBH₄ composite starts to release hydrogen from just 180 °C, which is 100 °C lower than the onset dehydrogenation temperature of ball milled LiBH₄. Moreover, the 30 mol% h-BN doped LiBH₄ composite can release 12.6 wt% hydrogen in 2 h at 400 °C, while only 0.98 wt% H₂ is gained from ball milled LiBH₄. The apparent activation energy (E_a) of hydrogen desorption had been reduced from 198.31 kJ mol⁻¹ for ball milled LiBH₄ to 155.8 kJ mol⁻¹ for 30 mol% h-BN doped LiBH₄. In addition, the rehydrogenation of the composite is achieved under 400 °C and 10 MPa of H₂. These remarkable results are largely attributed to the lone pair electrons of nitrogen induced destabilization of LiBH₄ and their heterogeneous nucleation.

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1. Introduction

Lithium borohydride (LiBH₄) has been widely investigated as a candidate for hydrogen storage materials due to its large hydrogen capacity (13.8 wt%) and volumetric hydrogen density (121 kg m⁻³).^1–3 The decomposition of LiBH₄ undergoes multiple stages,⁴ including polymorphic transformation at about 110 °C, melting at about 280 °C with a slight accompanying hydrogen desorption, and then the main evolution of hydrogen at 400–600 °C.^5,6 This major decomposition reaction is:⁷

LiBH₄ → LiH + B + 3/2H₂ (1)

Consequently, the desorption temperature of pristine LiBH₄ is higher than the desired value. Therefore, it is worthwhile to study strategies to reduce the thermodynamic stability and the hydrogen desorption kinetics of LiBH₄.

Since Züttel et al.¹ reported that LiBH₄ doped with SiO₂ desorbs hydrogen below 200 °C, a lower temperature than for bulk LiBH₄. Various potential dopants/catalysts, including metals, metal halides, and metal oxides, have been tentatively added into LiBH₄ to enhance its hydrogen storage properties. The metal, such as Mg,⁸ Al,⁹ Ni,¹⁰ Ti, V, Cr and Sc,¹¹ did not greatly improve the dehydrogenation properties of LiBH₄, indicating a temperature requirement higher than 400 °C for fast hydrogen desorption and slow dehydrogenation kinetics. Taking Al doped with LiBH₄ for example, 7.2 wt% hydrogen was released at 450 °C, and completing the dehydrogenation would take above 30 hours.^9,11,12 For the metal halides doped LiBH₄, the dehydrogenation temperature was considerably decreased to 220–300 °C by doped with FeCl₂, CoCl₂, NiCl₂,¹³ LaCl₃, CeCl₃, LaF₃ and CeF₃,¹⁴ and even to about 100 °C by mixing with TiF₃,¹⁵ TiCl₃ (ref. 16) and ZnF₂.¹⁷ However by-product including harmful diborane also will release, and thus results in drastically degraded reversible hydrogen capacity.¹⁸ By doping with metal oxides, such as Fe₂O₃, V₂O₅, Nb₂O₅, TiO₂,¹⁹ LiBH₄ was able to dehydrogenate at much lower temperatures.^20–22 For example, 6 wt% hydrogen could be released at 200 °C for a LiBH₄–Fe₂O₃ mixture with a mass ratio of 1 : 2.²⁰ Unfortunately the destabilization of LiBH₄ by metal oxides resulted from a redox reaction, which caused LiBH₄ could not be rehydrogenated under moderate conditions.²⁰

In this paper, the hydrogen storage properties of LiBH₄ defined by doping with h-BN were investigated. We observed that the dehydrogenation properties of LiBH₄ are improved by doping with h-BN. It is found that h-BN addition greatly facilitates the dehydrogenation kinetics of LiBH₄, and the rehydrogenation properties are also enhanced. This improvement may be correlated to the lone pair electrons of nitrogen induced destabilization of LiBH₄ and a heterogeneous nucleation process of the solid decomposition products of reaction (1) on the surface of BN.

2. Experimental section

Commercial hexagonal boron nitride (99%, Aladdin), and LiBH₄ (95%, Acros Organics) were used in this research without further purification. Samples of LiBH₄ and x h-BN (x = 0, 0.05, 0.15, 0.3, 0.5) were ball milled under a hydrogen pressure of 3 MPa for 3 h, The ball milling process was paused for 6 min every 24 min to avoid an increasing temperature of the samples. The as-synthesized samples were dehydrogenated at 400 °C under 0.1 MPa H₂ and rehydrogenated at 400 °C under 10 MPa of H₂. All experimental operations were performed in a high-pure Ar-filled glovebox.

The X-ray diffraction (XRD) analysis of the samples were performed on an X'Pert Pro (PANalytical, Netherlands) with Cu Kα radiation at 40 kV and 40 mA with the step size of 0.02° from 10° to 90° (2θ). Scanning electron microscopy (SEM, Hitachi SU-70) was performed to examine surface morphology of the samples. The microstructure was further examined by transmission electron microscopy (TEM, JEOL JEM-1200EX working at 200 kV) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-Twin working at 200 kV). The differential scanning calorimetry and mass spectrometer (DSC-MS) measurements were conducted on a synchronous thermal analysis (Netzsch STA 449F3 analyzer/Netzsch Q403C mass spectrometer) with a heating rate of 5 K min⁻¹ from room temperature to 500 °C under high purity argon condition with a purge rate of 50 ml min⁻¹. During the experiments the MS signals at m/z = 2, 17, and 27 were recorded to detect possible evolving gases H₂, NH₃ and B₂H₆. Hydrogen desorption and absorption properties of the samples were quantitatively examined with volumetric method by a carefully calibrated Sievert's type apparatus. In order to compare with the hydrogen capacity of bulk LiBH₄, the hydrogen contents presented in this work are calculated based only on the LiBH₄.

3. Results and discussion

Fig. 1(a) shows the XRD patterns of pristine and ball milled h-BN. For both the pristine h-BN and ball milled h-BN, all the detected peaks can be assigned to the diffraction of h-BN phase according to JCPDS 34-0421. The sharp and intensity of diffraction peaks in Fig. 1(a) indicate the good crystallinity of h-BN as-received sample. However, after ball milling at 400 rpm for 3 h, the diffraction peaks changed slightly, exhibited a little lower intensity and wider diffraction peaks. These may be ascribed to some amorphousization of h-BN happened, such as BN layers drifted during ball milling. The graphite-like structure of h-BN was studied by TEM and HRTEM, shown in Fig. 1(b) and (c). The TEM micrograph shows that the h-BN sample has a clear and flat surface. In the HRTEM image, the measured lattice fringes of ≈0.326 nm coincides with the lattice spacing of the (002) planes of the h-BN. The corresponding fast Fourier transform (FFT) analysis reveals hexagonal spots, indicating the structure of h-BN is hexagonal sheets.²³

Fig. 1 (a) XRD patter of h-BN, (b) TEM image of h-BN and (c) HRTEM images of h-BN.

Fig. 2 illustrates isothermal dehydrogenation curves of ball milled LiBH₄ and LiBH₄ doped with various amount of h-BN samples. The dehydrogenation properties of LiBH₄ were markedly improved upon ball milling with h-BN, even addition amount of h-BN is only 5 mol% of LiBH₄, the dehydrogenation behavior is quite different form pure LiBH₄. Obviously, it could be found that the improvement effect on dehydrogenation properties of LiBH₄ increases with addition amount of h-BN. When the h-BN doping amounts add up to 30 mol% of LiBH₄, 12.6 wt% hydrogen can be released within 2 h at 400 °C, while only 0.98 wt% hydrogen can be gained for ball milled LiBH₄. More importantly, ball milled LiBH₄ finishes the decomposition will take about 6000 min, and it can be shorted to 180 min after doping with 30 mol% h-BN. Therefore, it can be concluded that the doped h-BN greatly enhanced the dehydrogenation kinetics of LiBH₄.

Fig. 2 (a) Isothermal dehydrogenation curves of ball milled LiBH₄ and LiBH₄ doped with different amount of h-BN at 400 °C.

It is common knowledge that h-BN has very good chemical inertness, especially its resistance to acids and molten metals and its stability in air up to 1000 °C.^23,24 And h-BN does not react with LiBH₄ neither after ball milling at 400 rpm nor after dehydrogenation at 400 °C. It is confirmed by the XRD patterns of LiBH₄ doped with h-BN samples, as shown in Fig. 3. The diffraction peaks of the samples after ball milling are corresponding to starting materials (LiBH₄ and h-BN), and after dehydrogenation the diffraction peaks are assigned to h-BN and LiH. LiH is the product of LiBH₄ decomposition.

Fig. 3 XRD patterns of ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples: (a) after ball milling, (b) after dehydrogenation at 400 °C.

The dehydrogenation property of the ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples were investigated by DSC-MS as shown in Fig. 4. As displayed in the DSC profiles, both ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples exhibit of four endothermic peaks during the heating process to 500 °C at 5 °C min⁻¹. The first endothermic peak around 113 °C corresponds to the phase transition of LiBH₄, and the second one around 285 °C is from melting of LiBH₄. The other two peaks at higher temperatures (350–500 °C) are attributed to the decomposition of LiBH₄ with hydrogen releases.¹ Obviously, after doping with h-BN, the melting of LiBH₄ shifted to the lower temperature (276 °C), which may be due to LiBH₄ particle size reduces after ball milling with low density h-BN. The major dehydrogenation temperature of LiBH₄ reduces to 431 °C, lower than ball milled LiBH₄. From the MS profiles, only hydrogen was detected from the thermal desorption measurement by MS (data of possible B₂H₆, and NH₃ are shown in Fig. 4). The main hydrogen release peak are 467 °C and 435 °C for ball milled LiBH₄ and LiBH₄ doped with h-BN samples respectively, which is corresponding to the DSC results. With a careful analysis, we found that LiBH₄ begin releasing hydrogen at 180 °C, which is 100 °C lower than the onset dehydrogenation temperature of ball milled LiBH₄. We attribute the lower onset desorption temperature of LiBH₄ to the lone pair electrons of nitrogen atom on the h-BN surface, which may induce destabilization of LiBH₄ by the lone pair electrons complex with “electron-deficient” molecule LiBH₄.²⁵

Fig. 4 DSC-MS profiles of ball milled LiBH₄ and 30 mol% doped h-BN LiBH₄ samples at a heating rate of 5 K min⁻¹.

In order to gather better insight of the improvement on dehydrogenation kinetics of 30 mol% h-BN doped LiBH₄, apparent activation energy (E_a) related to the dehydrogenation of LiBH₄ was quantitatively determined by using the Kissinger's method according to reaction (2):²⁶

ln(β/T_p²) = −E_a/RT_p + ln(AR/E_a) (2)

where β (K min⁻¹) is the heating rate, T_p (K) is the absolute temperature at the maximum decomposition rate, E_a is the activation energy, A is the pre-exponential factor and R (8.314 J mol⁻¹ K⁻¹) is the gas constant. In this work, T_p (K) are collected from the DSC curves with different rates for 5, 7.5, 10, 12.5, 15 K min⁻¹, as shown in (ESI) Fig. S1.† Thus, the activation energy, E_a, can be obtained from the slope in a plot of ln(β/T_p²) vs. 1000/T_p. The apparent activation energy for the ball milled LiBH₄ dehydrogenation is estimated to 198.31 kJ mol⁻¹, which is in agreement with the previous report.^27,28 The apparent activation energy of 30 mol% h-BN doped LiBH₄ dehydrogenation is estimated to be 155.80 kJ mol⁻¹, significantly lower than 198.31 kJ mol⁻¹ of ball milled LiBH₄ (as shown in Fig. 5). These results provide quantitative evidence for h-BN additive can effectively improve the hydrogen desorption kinetics of LiBH₄.

Fig. 5 Kissinger's plots obtained from the DSC data for the dehydrogenation of ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples.

The products of ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples after dehydrogenation were subsequently rehydrogenated at 400 °C under 10 MPa H₂ for 12 h. And the XRD patterns of rehydrogenation samples are presented in Fig. 6(a). The diffraction peaks of LiBH₄ doped with 30 mol% h-BN rehydrogenated samples are assigned to LiBH₄, h-BN and LiH, indicating that LiBH₄ was regenerated upon hydrogenation, even though the reverse reaction proceeded incompletely, due to the sequaration of a part of LiH and boron in the material after dehydrogenation.²⁷ However, without doping with h-BN, the diffraction peaks of LiBH₄ cannot be found in the rehydrogenation sample under the same condition, but some unknown and LiH diffraction peaks could be detected. The rehydrogenation of LiBH₄ has been reported only under harsh conditions such as 600 °C and 35 MPa H₂.^29,30 Meanwhile the rehydrogenation capacity of h-BN doped LiBH₄ sample after rehydrogenation at 400 °C under 10 MPa H₂ for 12 h was performed, as seen in Fig. 6(b). It could be found that over 7.0 wt% hydrogen could be released in the second dehydrogenation for the 30 mol% h-BN doped LiBH₄ sample. And the rehydrogenation capacity of the composites is about 6 wt% during the third dehydrogenation. In other words, experimental results of this work showed that the rehydrogenation properties of LiBH₄ were improved by h-BN addition in terms of rehydrogenation conditions and rehydrogenation percentage than that of ball milled LiBH₄.

Fig. 6 XRD patterns and isothermal dehydrogenation curves of ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples after rehydrogenation under 400 °C and 10 MPa of H₂ for 12 h.

In the SEM examinations of ball milled LiBH₄ and 30 mol% h-BN doped LiBH₄ samples, appreciable difference on the LiBH₄ particle morphology was observed between ball milled LiBH₄ and LiBH₄ doped with h-BN. As shown in Fig. 7(a), ball milled LiBH₄ particles are not small. Even after ball milling for 3 h, the particles size is over 2 μm, because of the agglomerations reunited after ball milling. Interestingly, after ball milling with 30 mol% h-BN, LiBH₄ particles become much smaller than 2 μm, dispersing on h-BN flat surface, as shown in Fig. 7(c). It is believed that decreasing particle size is beneficial to enhance dehydrogenation of LiBH₄.^31,32 Fig. 7(b) and (f) show the SEM images of ball milled LiBH₄ and 30 mol% doped LiBH₄ after dehydrogenation at 400 °C. Obviously, after dehydrogenation, the ball milled LiBH₄ decomposition product (B and LiH) particles agglomerated together to form larger clusters. For 30 mol% h-BN doped LiBH₄ sample, the decomposition product particles size does not increase. In this regard, the h-BN additives might play a role as effect heterogeneous nucleation site for the decomposition and prevent the nucleated particles from further growth during dehydrogenation reaction.³³ These might be largely credible explanation for why the dehydrogenation and rehydrogenation properties of LiBH₄ are improved by doping with h-BN.

Fig. 7 SEM images of (a) ball milled LiBH₄ sample, (b) ball milled LiBH₄ sample after dehydrogenation at 400 °C, (c) neat h-BN, (d) 30 mol% h-BN doped LiBH₄ sample after ball milling and (f) 30 mol% h-BN doped LiBH₄ sample after dehydrogenation at 400 °C.

Based on the above analyses, the whole process of synthesis and de/rehydrogenation for LiBH₄ doped with h-BN systems from a micro point of view is illustrated in Scheme 1. After LiBH₄ ball milled with h-BN sheets, LiBH₄ particles become small and well disperse on the surface of h-BN. The lone pair electrons of N atom on the surface of h-BN induces part of LiBH₄ destabilization, resulting in the onset of dehydrogenation of LiBH₄ was decreased to 180 °C with a small amount of hydrogen desorption, which also could be supported by the dehydrogenation of LiBH₄ doped with graphite as shown in the ESI Fig. S2† During the dehydrogenation process, the LiBH₄ covered on the surface of h-BN will decomposes firstly, and then the products of LiBH₄ decomposition (LiH and B particles) play the role of the nucleation sites. And with effect heterogeneous nucleation site of h-BN, the product particles of LiBH₄ decomposition are more inclined to dispersing on the surface of h-BN. In the rehydrogenation process, the composites can reverse to form LiBH₄ in short distance on the surface of h-BN.

Scheme 1 Illustration of the process of synthesis and de/rehydrogenation for LiBH₄ doped with h-BN systems.

4. Conclusions

The effects of h-BN additives on the dehydrogenation and rehydrogenation properties of LiBH₄ were investigated. The results show that the dehydrogenation kinetics of LiBH₄ is greatly improved by doping with h-BN. For instance, 30 mol% h-BN doped LiBH₄ can release 12.6 wt% hydrogen in 2 h at 400 °C, while only 0.98 wt% H₂ is gained for ball milled LiBH₄. The onset dehydrogenation temperature of LiBH₄ is reduced to 180 °C from 280 °C of ball milled LiBH₄, the apparent activation energy of hydrogen desorption has been reduced from 198.31 kJ mol⁻¹ for ball milled LiBH₄ to 155.8 kJ mol⁻¹ for LiBH₄ doped with 30 mol% h-BN. Meanwhile, over 7.0 wt% hydrogen can be rehydrogenated at 400 °C under 10 MPa H₂. XRD, SEM and TEM analyses demonstrate that h-BN remain stable in the ball milling process and de/rehydrogenation. It is believed that h-BN dopant services as effect heterogeneous nucleation site for dehydrogenation of LiBH₄ and plays a role in preventing the nucleated particles from further growth. Besides, the presupposition complex reaction between the lone pair electrons of N atom on surface of h-BN with “electron-deficient” molecule LiBH₄ induces destabilization of LiBH₄ might take effect on improving the dehydrogenation of LiBH₄.

Acknowledgements

The authors gratefully acknowledge the financial supports for this research from the National High Technology Research & Development Program of China (2012AA051503), the National Natural Science Foundation of China (51471151, 51171173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), and the Zhejiang Provincial Science & Technology Program of China (2014C31134).

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Footnote

† Electronic supplementary information (ESI) available: DSC curves with different rates and isothermal dehydrogenation curves of the LiBH₄ doped with graphite. See DOI: 10.1039/c5ra05438a

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