Dehydrogenation mechanism of LiBH4 by Poly(methyl methacrylate)

doi:10.1016/j.jallcom.2014.12.268

Journal of Alloys and Compounds

Volume 645, Supplement 1, 5 October 2015, Pages S100-S102

https://doi.org/10.1016/j.jallcom.2014.12.268 Get rights and content

Highlights

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LiBH₄ is amorphous after modified with PMMA.
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Dehydrogenation temperature of LiBH₄ decreases by 120 °C after modifying with PMMA.
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The LiBH₄@PMMA composite releases 10 wt.% hydrogen at 360 °C within 1 h.
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CO group of PMMA weakens the BH bonds to lower dehydrogenation temperature.

Abstract

We investigated the dehydrogenation properties and mechanism of Poly(methyl methacrylate) (PMMA) confined LiBH₄. Thermal stability of LiBH₄ was reduced by PMMA, with a decrease in dehydrogenation temperature by 120 °C. At 360 °C, the composite showed fast dehydrogenation kinetics with 10 wt.% of hydrogen released within 1 h. The improved dehydrogenation performance was mainly attributed to the reaction between LiBH₄ and PMMA forming Li₃BO₃ as a final product. Furthermore, the presence of electrostatic interaction between B atom of LiBH₄ and O atom in the carbonyl group of PMMA may weaken the B single bond H bonding of [BH₄]⁻ and lower the hydrogen desorption temperature.

Introduction

Hydrogen is considered as a promising alternative energy carrier owing to its high-energy density, abundance, light weight and pollution-free burning [1]. Developing a safe and efficient hydrogen storage material is one of the key challenges for the mobile application of hydrogen [2], [3], [4]. Due to the high gravimetric (18.5 wt.%) and volumetric (121 kg H₂/m³) hydrogen density, lithium borohydride (LiBH₄) has been acknowledged as a potential candidate for hydrogen storage materials [5], [6], [7]. However, due to the unfavorable high thermal stability (e.g. decomposition peak temperature of ∼470 °C), the practical utilization of LiBH₄ as hydrogen storage medium is hampered [8]. Hence, several approaches including reactant destabilization, catalyst/additive introduction, nanostructuring, and anion/cation substitution have been applied to decrease the dehydrogenation temperature and accelerate the kinetics [7], [9].

Nanoconfinement is a viable way to improve the dehydrogenation performance by decreasing diffusion path lengths and increasing surface areas [10], [11], [12], [13], [14], [15], [16]. For example, confining LiBH₄ in highly ordered nanoporous carbon with 2 nm average diameter was reported to decrease the onset desorption temperature from 460 to 220 °C [12]. Recently, we encapsulated LiBH₄ in a flexible material, PMMA, where the onset hydrogen evolution temperature was reduced close to room temperature [17]. Furthermore, the confinement in PMMA improves the air resistance of LiBH₄ due to the protection from gaseous O₂ and H₂O.

In present work, we investigated the dehydrogenation mechanism of the PMMA confined LiBH₄. Li₃BO₃ was observed as a final product, indicating that the improved dehydrogenation performance was mainly attributed to the reaction between LiBH₄ and PMMA.

Section snippets

Experimental

Lithium borohydride solution (2.0 M LiBH₄ in tetrahydrofuran) (abbreviate as LiBH₄/THF here and after) was purchased from Sigma–Aldrich Co. PMMA (MW 120,000) was supplied by Alfa-Aesar. All these reagents were used without any further purification and were stored and handled in a glove box equipped with an Ar recirculation system with water and hydrogen below 3 ppm so that prevent them from oxidation. In a typical experiment, 5 ml LiBH₄/THF solution containing 0.218 g of LiBH₄ was added to 0.1452 g

Phase structure and morphology

X-ray diffraction (XRD) patterns of pure LiBH₄, PMMA and 60LP are shown in Fig. 1a. PMMA shows a broad halo at 2θ = 17° and LiBH₄ shows a low-temperature (orthorhombic) phase. No reflections of LiBH₄ were observed in 60LP, implying that LiBH₄ is in amorphous state. In Fig.1b, the SEM image of 60LP shows a porous structure with pore diameters ranging from 60 to 300 nm, implying that the sizes of LiBH₄ particles in 60LP are within 60 to 300 nm.

Dehydrogenation properties

The dehydrogenation properties of PMMA, pure LiBH₄ and

Conclusion

The PMMA confined LiBH₄ was successfully prepared by a solution method. By confinement in PMMA, the temperature of major hydrogen desorption of LiBH₄ was reduced to 350 °C and faster desorption kinetics was achieved, e.g., 11 wt.% of hydrogen released within 5 h. The enhanced dehydrogenation performance is mainly attributed to the formation of Li₃BO₃. Electrostatic effect between B atom of LiBH₄ molecule and O atom in C double bond O group of PMMA may weaken the B single bond H bonding and lower the dehydrogenation

Acknowledgements

This work was supported by the National Natural Science Foundation of China Projects (Nos. 51431001, U1201241 and 51271078), the Key Project of DEGP (cxzd1010) and by GDUPS (2014).

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Dehydrogenation mechanism of LiBH4 by Poly(methyl methacrylate)

Highlights

Abstract

Introduction

Section snippets

Experimental

Phase structure and morphology

Dehydrogenation properties

Conclusion

Acknowledgements

Int. J. Hydrogen Energy

J. Power Sources

J. Alloys Comp.

Int. J. Hydrogen Energy

Acta Mater.

Int. J. Hydrogen Energy

Nature

Chem. Soc. Rev.

Chem. Rev.

Dehydrogenation mechanism of LiBH₄ by Poly(methyl methacrylate)