Effects of helical GNF on improving the dehydrogenation behavior of LiMg(AlH4)3 and LiAlH4

The present paper reports the effect of graphitic nanofibres (GNFs) for improving the desorption kinetics of LiMg(AlH₄)₃ and LiAlH₄. LiMg(AlH₄)₃ has been synthesized by mechano-chemical metathesis reaction involving LiAlH₄ and MgCl₂. The enhancement in dehydrogenation characteristics of LiMg(AlH₄)₃ has been shown to be higher when graphitic nanofibres (GNFs) were used as catalyst. Out of two different types of nanofibres namely planar graphitic nanofibre (PGNF) and helical graphitic nanofibre (HGNF), the latter has been found to act as better catalyst. We observed that helical morphology of fibres improves the desorption kinetics and decreases the desorption temperature of both LiMg(AlH₄)₃ and LiAlH₄. The desorption temperature for 8 mol% HGNF admixed LiAlH₄ gets lowered from 159 °C to 128 °C with significantly faster kinetics. In 8 mol% HGNF admixed LiMg(AlH₄)₃ sample, the desorption temperature gets lowered from 105 °C to ∼70 °C. The activation energy calculated for the first step decomposition of LiAlH₄ admixed with 8 mol% HGNF is ∼68 kJ/mol, where as that for pristine LiAlH₄ it is 107 kJ/mol. The activation energy calculated for as synthesized LiMg(AlH₄)₃ is ∼66 kJ/mol. Since the first step decomposition of LiMg(AlH₄)₃ occurs during GNF admixing, the activation energy for initial step decomposition of GNF admixed LiMg(AlH₄)₃ could not be estimated.