Structure determination of H-encapsulating clathrate compounds in aberration-corrected STEM

Abstract

In the quest for alternative energy sources, hydrogen has long been touted as one of the most likely candidates to replace fossil fuels, provided practical solutions for its storage and transport can be found. Clathrate hydrates, amongst other nano-porous materials, have shown remarkable potential for hydrogen storage, adsorbing up to 7.5%wt H, albeit in extreme pressure and temperature conditions [1]. Upon crystallising, these compounds form a three-dimensional host matrix where guest atoms can be accommodated within distinct polyhedral “cages”. Recent work exhibited a sodium silicide clathrate Na_xSi₄₆, consisting of a silicon matrix with two distinct Na guest sites, labelled 2a and 6d, stable at room temperature and with promising hydrogen encapsulation properties [2]. Magic angle spinning nuclear magnetic resonance was initially used to show that in the growth conditions specific to this work some of the 6d sites were Na-deficient and H-rich.