Synthesis of helical carbon nanofibres and its application in hydrogen desorption

In this communication, we report the synthesis of helical carbon nanofibres (HCNFs) by employing hydrogen storage intermetallic LaNi5 as the catalyst precursor. It was observed that oxidative dissociation of LaNi5 alloy (2LaNi5 + 3/2O2 → La2O3 + 10Ni) occurred during synthesis. The Ni particles obtained through this process instantly interacted with C2H2 and H2 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 m2/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 (NaAlH4). We have found that the present as-synthesized HCNFs, with metallic impurities, indeed work as an effective catalyst. The pristine NaAlH4 and 8 mol% as-synthesized HCNFs admixed NaAlH4, 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 LaNi5 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, NaAlH4.