Synthesis of hollow carbon nano-onions and their use for electrochemical hydrogen storage

In this study, we report an efficient method for synthesis of well-graphitized hollow carbon nano-onions (CNOs). CNOs were firstly fabricated by chemical vapor deposition (CVD) method at 850 °C using an Fe–Ni alloy catalyst with diameters of 10–15 nm. Then hollow CNOs were obtained by annealing as-prepared CNOs at 1100 °C for 3 h. It is found that during the CVD growth, the presence of nickel retards the deactivation of Fe–Ni–C austenite, providing the possibility for the growth of up to two hollow CNOs from each alloy particle. The subsequent high-temperature annealing led to the escaping of the Fe–Ni alloy from the graphitic layers, and the re-catalysis of precipitation and graphitization of the carbon atoms previously dissolved in the alloy particle (Fe_0.64Ni_0.36) to form hollow CNOs. The hollow CNOs exhibit good performance as materials for electrochemical hydrogen storage, with a discharge capacity of 481.6 mAh/g under a current density of 500 mA/g, corresponding to a hydrogen storage capacity of 1.76 wt.%. Our results demonstrate that the hollow CNOs are promising materials as a storage medium for hydrogen as a fuel source.