The effect of Li-doping and doping methods to hydrogen storage capacities of some carbonaceous materials

Today, the storage capacities of carbonaceous materials used for effective and efficient storage of hydrogen are not yet at the desired level, and studies should be conducted to improve the storage capacities of these materials. Therefore, the aims of this study are to investigate i. the effect of lithium doping according to the solvent removal method on the hydrogen storage capacities of graphene, single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT) and fullerene (C70) and to compare ii. the effect of solvent removal and mechanical Li-doping methods on the storage capacity of fullerene. Surface areas of graphene and fullerene increased with lithium doping, while those of carbon nanotubes decreased. Graphene and carbon nanotubes have a porous structure, while fullerene has a very low pore volume. The band observed at 667 cm-1 in the FTIR spectrum of carbonaceous materials is an indication that all structures are doped with lithium. Raman analyses showed that Li-doping did not cause any significant changes in the structure of the other samples except for the mechanically Li-doped fullerene. Raman bands were not observed in the structure of the mechanically Li-doped fullerene. The hydrogen storage capacity of carbonaceous materials increased at low temperatures and high pressures. While graphene and SWCNT with high surface area have high hydrogen storage capacity, fullerene has low hydrogen storage capacity. At 20 bar and 77 K, SWCNT (1.605 wt%) and Li-doped graphene (1.416 wt%) had the highest adsorption capacity. Lithium-doping of fullerene by mechanical method further increased the hydrogen storage capacity. The increase in hydrogen storage capacity is more consistent with the pore volume than the BET surface area. The experimental data are consistent with the Freundlich isotherm at room temperature, while at cryogenic temperature the fullerene samples are consistent with the Freundlich isotherm and the graphene, SWCNT and MWCNT samples are consistent with the Langmuir isotherm. The low heat of adsorption confirmed that the interactions between carbonaceous materials and hydrogen are physically based. The pseudo second order kinetic equation showed very well agreement with the experimental data. Van der Waals interactions become important as the temperature decreases.