Synthesis of helical carbon nanofibres and its application in hydrogen desorption

Abstract

In this communication, we report the synthesis of helical carbon nanofibres (HCNFs) by employing hydrogen storage intermetallic LaNi 5 as the catalyst precursor. It was observed that oxidative dissociation of LaNi 5 alloy (2LaNi 5 + 3/2O 2 → La 2O 3 + 10Ni) occurred during synthesis. The Ni particles obtained through this process instantly interacted with C 2H 2 and H 2 gases, and fragmented to nanoparticles of Ni (∼150 nm) with polygonal shape. These polygonal shapes of Ni nanoparticles were decisive for the growth of helical carbon nanofibres (HCNFs) at 650 °C. TEM, SAED and EDAX studies have shown that HCNFs have grown on Ni nanoparticles. Typical diameter and length of the HCNFs are ∼150 nm and 6–8 μm respectively. BET surface area of these typical HCNFs has been found to be 127 m 2/g. It was found that at temperature 750 °C, spherical shapes of Ni nanoparticles were produced and decisive for the growth of planar carbon nanofibres (PCNFs). The diameter and length of the PCNFs are ∼200 nm and 6–8 μm respectively. In order to explore the application potential of the present as-synthesized CNFs, they were used as a catalyst for enhancing the hydrogen desorption kinetics of sodium aluminum hydride (NaAlH 4). We have found that the present as-synthesized HCNFs, with metallic impurities, indeed work as an effective catalyst. The pristine NaAlH 4 and 8 mol% as-synthesized HCNFs admixed NaAlH 4, at 160 °C–180 °C and for the duration of 5 h, liberate 0.8 wt% and 4.36 wt% of hydrogen, respectively. Thus there is an enhancement of ∼5 times in kinetics when as-synthesized HCNFs are used as the catalyst. To the best of our knowledge, the use of hydrogen storage alloy LaNi 5 as the catalyst precursor for the growth of HCNFs has not yet been done and thus represents a new feature relating to the growth of HCNFs. Furthermore, we have shown that the as-synthesized HCNFs work as an effective new catalyst for improving the dehydrogenation kinetics of the complex hydride, NaAlH 4.