Magnesium-graphene interphase boundaries created by high-pressure torsion enhance hydrogen storage kinetics:Mechanisms and significance of activation energy and frequency factor

TL;DR

Creating magnesium-graphene interphase boundaries through high-pressure torsion improves hydrogen storage kinetics by reducing grain size and increasing interface sites, without changing activation energy.

Contribution

This work demonstrates that interphase boundary engineering via severe plastic deformation enhances hydrogen storage kinetics and air resistance in magnesium-based materials.

Findings

Magnesium grain size reduced from 1 mm to 850 nm by HPT.

Graphene addition further refines grains to 10-500 nm.

Activation energy remains at 145 kJ/mol regardless of boundary presence.

Abstract

A strategy to overcome sluggish hydrogenation/dehydrogenation of magnesium is demonstrated by creating magnesium-graphene interphase boundaries via high-pressure torsion (HPT). HPT reduces the grain size of pure magnesium from 1 mm to 850 nm, with 70% of grain boundaries having high misorientation angles. Graphene addition leads to even finer grain sizes of 10-500 nm with a bimodal morphology. The magnesium-graphene composites exhibit superior kinetics at 623 K while maintaining high air resistance. Kinetic modeling reveals that the rate-controlling mechanism transits from interfacial reaction in coarse-grained magnesium to atomic diffusion in magnesium-graphene nanocomposites. Kissinger analysis shows that the activation energy for hydrogen desorption remains unchanged at 145 +/- 2 kJ/mol, regardless of the presence of grain or interphase boundaries. However, the frequency factor (number of successful attempts to overcome the activation energy) increases with the generation of interfaces, which serve as sites for hydrogen diffusion and heterogeneous metal/hydride nucleation. These findings highlight the impact of interphase boundary engineering via severe plastic deformation for enhancing the kinetics and air resistance of hydrogen storage materials.