Effects of chemical/structural characteristics of carbon scaffold on magnesium nanoconfinement and hydrogen storage properties

Abstract

Magnesium-based hydrogen storage has been studied a lot as a solid hydrogen storage method on account of advantages such as high theoretical hydrogen storage capacity, reversibility, and low density. However, commercialization of magnesium-based hydrogen storage is not easily achieved due to sluggish hydrogen ab/desorption kinetics, high reaction enthalpy, low oxidation stability, and difficulty in heat transfer management. To resolve these issues, magnesium has been physically constrained to various carbon scaffolds, forming magnesium nanoparticles and providing shortened hydrogen diffusion path by the nanoscale particles. In addition, the selective permeability to hydrogen gas of the carbon structures can improve low oxidation stability, and their catalytic effects induced by hetero atoms can also be expected. However, until now, many efforts have been conducted on nanoconfinement of magnesium in various carbon scaffolds, the confined Mg content in the carbon scaffold is very low, resulting in an inevitable loss in hydrogen storage capacity. Moreover, theoretical/experimental interpretations for correlation between chemical/physical properties of carbon structures and the evolution of magnesium nanocrystals and their hydrogen storage properties are insufficient. In this thesis, when magnesium nanocrystals are constrained to two-dimensional graphene oxide, the change in the evolution of magnesium nanoparticles according to the defect density including oxygen functional groups in graphene oxide, and the resulting effects on hydrogen storage properties were confirmed. In addition, it was demonstrated through DFT calculation that the higher the defect density of graphene oxide, the more favorable the formation of nanoscale magnesium clusters, discovering differences in magnesium nanoconfinement depending on the chemical properties of carbon scaffolds. The change in the formation behavior of magnesium nanoparticles according to the defect density may affect the hydrogen storage performance. Graphene oxide with a high defect density showed a fast hydrogen ab/desorption rate by forming uniform magnesium nanoparticles with 3-4 nm on the graphene surface, whereas reduced graphene oxide with a high defect density produced more aggregated magnesium particles that were not surrounded by graphene nanosheets, showing a relatively poor hydrogen storage performance. In addition, since the reduced graphene oxide has improved thermal conductivity according to the degree of reduction, it may affect the reaction induction time in the emission reaction, which is an endothermic reaction. On the other hand, by forming magnesium nanoparticles in a three-dimensional (3D) hollow carbon scaffold, the effect of nanoconfinement according to the structural properties was confirmed. Compared to the 2D carbon scaffolds, the 3D carbon scaffolds are more advantageous for the nano-compartmentalization of magnesium nanoparticles, and it is easier to control the size of magnesium nanoparticles by tuning the pore size. In particular, in this study, the pore properties were changed by controlling the pore size/volume and carbon layer thickness of the hollow carbon, and the magnesium nanoconfinement behaviors and hydrogen storage characteristics were confirmed according to these pore characteristics. Contrary to the reported nano-size effect, the case with a large cavity size, which was expected to be the most unfavorable for the size of magnesium nanoparticles, showed the fastest hydrogen ab/desorption performance. This can be attributed to the fact that, in addition to the nano-size effect, the distinct chemical/physical interactions with magnesium according to the pore characteristics of the carbon scaffold affected the hydrogen storage performance. Through this dissertation, it was revealed that the change in the chemical/structural properties of the carbon scaffold is an important factor in the formation of magnesium nanoparticles. Moreover, it is expected that it will be possible to design a carbon scaffold for effective nanoconfinement of solid hydrogen storage materials.