Kinetic- and thermodynamic-based improvements of lithium borohydride incorporated into activated carbon

Abstract

LiBH₄ was incorporated into an activated carbon (AC) scaffold using a chemical impregnation method. Confinement of LiBH₄ within the carbon nanopores was found to significantly improve both the hydrogen sorption kinetics and thermodynamics, compared to the bulk hydride. The LiBH₄/AC sample starts to release hydrogen from just 220 °C, which is 150 °C lower than the onset dehydrogenation temperature of bulk LiBH₄. The dehydrogenation rate of the LiBH₄/AC sample was one order of magnitude faster than that of bulk LiBH₄. The temperature and hydrogen pressure conditions required for restoring the hydride were also significantly reduced when LiBH₄ was incorporated into AC. Preliminary study showed that the dissociation hydrogen pressure of LiBH₄ could be enhanced by around one order of magnitude upon incorporating the hydride into AC. X-ray diffraction, Fourier transform infrared spectroscopy and nitrogen adsorption analyses were used to confirm the nanostructure of LiBH₄ in the AC scaffold.

Introduction

The lack of safe and efficient means for onboard hydrogen storage is one of the major challenges facing the commercialization of hydrogen-powered vehicles [1]. Extensive research efforts for decades on traditional metal/alloy hydrides and nanostructured carbon materials have led to no viable hydrogen storage system that can reversibly store over 6 wt.% hydrogen at a temperature relevant to the practical operation of a proton exchange membrane fuel cell. Recently, light metal complex hydrides that contain ($\text{AlH}_4{}^{-}$) [2], [3], [4], [5], ($\text{NH}_2{}^{-}$) [6], [7], [8], [9] or ($\text{BH}_4{}^{-}$) [10], [11], [12], [13], [14], [15], [16], [17] anionic units have received considerable interest as potential high-capacity hydrogen storage media. However, the reversible dehydrogenation of these complex hydrides is restricted by the problematic H-exchange kinetics and thermodynamics imposed by their strong and highly directional covalent and/or ionic bonds [18], [19].

Several approaches have been described in the literatures to address the kinetic and thermodynamic limitations of complex hydrides. Catalyst doping has been demonstrated as an effective method for improving hydrogen sorption kinetics of alanate systems [2], [3], [4], [5]. Efforts using destabilization strategies have produced 2LiNH₂+MgH₂ [7], [8] and 2LiBH₄+MgH₂ [12], [13] systems with favorable thermodynamics. However, a viable hydrogen storage material requires favorable properties both kinetically and thermodynamically to allow dehydrogenation and re-hydrogenation reactions to proceed at moderate temperatures. In this regard, reducing the dimensions of hydride materials to the nanoscale is considered to be a promising approach [20], [21]. However, the practical application of nanoscale hydride materials is hindered by a lack of suitable preparation technologies, and by grain growth and particle agglomeration occurring during dehydrogenation/hydrogenation cycles at elevated temperatures [20].

Recently, the use of foreign structure-directing agents has been developed as a novel method for preparing nanoscale hydride materials and for preserving their nanostructure during dehydrogenation/hydrogenation cycles. Autrey et al. incorporated ammonia borane (NH₃BH₃) into mesoporous silica [22] and carbon aerogel [23] scaffolds. As a result of the tailored nanostructure of the hydride, both the dehydrogenation kinetics and thermodynamics were significantly improved, compared to the bulk material. Similarly, Schüth et al. encapsulated sodium alanate (NaAlH₄) in porous matrices [24]; de Jong et al. prepared supported NaAlH₄ nanoparticles on surface-oxidized carbon nanofibers [25]. Both materials exhibited improved hydrogen sorption kinetics relative to bulk NaAlH₄. Vajo and co-workers extended this nanoengineering method to the preparation of nanophase LiBH₄. By using a melt infiltration technique, they succeeded in incorporating the hydride into nanoporous carbon aerogels, which resulted in substantially improved H-exchange properties of LiBH₄ [26], [27]. These studies have demonstrated that creating nanophase hydrides using porous scaffolds is a promising route to improve hydrogen storage properties of noninterstitial hydride materials. However, the effect of particle size on the thermodynamics of complex hydrides remains unclear. In part, there is a lack of evidence to confirm the incorporation of the light metal hydrides and their states in the nanoporous scaffolds as the weak scattering of electrons by the light component elements restricts the direct observation of hydride nanoparticles by electron microscopy.

In this paper, we report that LiBH₄, a material with extremely high hydrogen content (18.5 wt.% and 0.124 kg l⁻¹), can be readily incorporated into nanoporous activated carbon (AC) using a simple chemical impregnation technique. AC was selected as the structure-directing agent due to its chemical inertness, nanoporous texture, commercial availability and low cost. Our study shows that, compared to bulk LiBH₄, the prepared LiBH₄/AC nanocomposite exhibits significantly improved H-exchange properties, both kinetically and thermodynamically. The improvements in H-exchange properties are discussed with reference to structural characterization of LiBH₄ confined within the nanoporous AC scaffold.