Catalytically enhanced hydrogen storage performance of heteroatom-modified graphene/magnesium nanocrystal composites

Abstract

Magnesium hydride has been extensively studied for solid-state hydrogen storage because of its high storage density (7.6 wt.%) and its abundance in the earth’s crust

however, strong binding enthalpies, sluggish kinetics, and poor oxidative stability impede their practical use. Among different strategies to overcome these limitations, encapsulating metal hydrides with graphene derivatives can be pragmatic since it allows metal hydrides to have an air-stability due to its hydrogen-selective permeability, simultaneously offering a catalytic function. Additionally, graphene derivatives limit the growth of magnesium crystals to few nanometers in their size. In this study, nitrogen elements were doped over graphene layers by thermal annealing of reactants at five temperatures to boost hydrogen storage kinetics of graphene/magnesium nanocrystal composites. By conducting X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), it was revealed that the annealing temperatures play an important role in determining structure and doping density of N-doped graphenes. Heteroatom doped graphene acts not only as a support but also as a catalyst for (de)hydrogenation, which can be confirmed by a reduction of hydrogen absorption activation energy. As a result of the optimization of the annealing temperature, the N-doped graphene annealed at 650 $^\circ C$/Mg nanocrystal composite (N650-rGO-Mg) exhibited the best hydrogen storage kinetics, and a mechanism of such enhancement was studied by absorption/desorption activation energy calculations. It was concluded that pyridinic nitrogen functionality contributes to the enhancement of hydrogen storage kinetics the most by donating its electron density to magnesium nanocrystals.