Sorption properties of nanostructured ball-milled porous silicon for solid-state hydrogen storage up to 80 bar

Abstract

Solid-state storage is a feasible solution to store hydrogen compared to commercially available techniques. The disadvantage of using metal and complex hydrides for storage is the elevated temperature operation (>400 °C) and slow reaction kinetics. Porous materials like carbon nanostructures, metal-organic frameworks, zeolites, and porous polymers possess high surface energy and hydrogen affinity. However, cryogenic temperature operation and low storage at ambient conditions are major limitations for practical storage applications. Modifying the material morphology and storage parameters to enhance hydrogen storage is under intense investigation. This study focuses on nanostructuring porous silicon (PS), evaluating its structural characteristics, and investigating its ability to store hydrogen at up to 80 bar. The challenges of large particle size and low-pressure hydrogen storage in the novel porous material is addressed with possible mechanisms. The discussion explored the potential of utilizing interconnected pores via nanoscale engineering and increased charging pressure to optimize hydrogen exposure. Nanostructuring the hand-grinded porous Si (HGPS) reduces crystallite size, boosts surface energy and enhances thermodynamics. At 80 bar and 120 °C, the ball-milled PS (BMPS) exhibits a hydrogen storage capacity of 10.7 wt%. The isosteric heat of adsorption is utilized to optimize storage conditions to achieve useable capacity. The hydrogen adsorption pressure is optimized between 40 and 60 bar, where the storage capacity ranges from 2 wt% and 6 wt% (meeting the hydrogen storage target set by the US Department of Energy). X-ray Photoelectron Spectroscopy investigates surface states and bonding involving silicon hydrides. The effect of nanostructuring on decomposition energy is observed by differential scanning calorimetry. The decreased crystallite size, examined through X-ray diffraction and Raman spectroscopy, exposes nanopores accessible to hydrogen. The accessibility enhances storage capacity in free and surface-affixed states, rendering BMPS for reversible storage applications. The lower temperature requirement for hydrogen adsorption and release from storage material aligns with Sustainable Development Goal 7 (SDG 7) on affordable and clean energy.