Physical Adsorption for Hydrogen Storage

Hydrogen can be physically adsorbed onto the surface of porous materials like metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). Adsorption occurs through weak van der Waals forces between the hydrogen molecules and the surface of the material. MOFs and COFs offer extremely large internal surface areas for adsorption due to their nanoporous structures. Some promising MOFs and COFs have been shown to adsorb 5-7% of their weight in hydrogen by physisorption alone. However, further improvements are still needed to meet the DOE system-level storage targets of 5.5 wt% and 40 g H2/L. Research efforts continue to design new frameworks with optimized pore sizes and chemistries to improve adsorption capacities.

Hydro Storage in Nanotubes and Nanofibers

Carbon nanotubes (CNTs) and metal nanofibers are also being investigated for Hydrogen Storage through adsorption. The one-dimensional cavities within CNTs provide confinement effects that can enhance hydrogen binding compared to other carbon scaffolds. Some studies have reported gravimetric adsorption capacities approaching 1 wt% H2 in CNT bundles at 77 K and moderate pressures. Doping CNTs with light elements like boron and nitrogen can further boost capacities, with a recent work achieving 1.3 wt% H2 storage in N-doped CNTs. Metal nanofibers incorporating magnesium, lithium or other light-element alloys have also shown promise, storing 5-6 wt% H2 through physisorption or dissolution into the bulk. However, reversibility and thermodynamics need improvement for practical applications.

Chemisorption in Metal Hydrides

Another technique for storing hydrogen is through chemisorption in complex metal hydrides. In this process, hydrogen forms new strong chemical bonds with the host metals. Some representative complex hydrides currently under investigation include sodium alanate (NaAlH4), ammonia borane (NH3BH3), and magnesium hydride (MgH2). These materials can store hydrogen at weights ranging from 5-20 wt%, significantly exceeding the DOE system targets. However, challenges remain in terms of the high temperatures (100-300°C) required for hydrogen absorption/desorption and their sluggish sorption kinetics at milder conditions. Doping and nanoconfinement are strategies being explored to reduce the operating temperatures and accelerate sorption kinetics in metal hydrides.

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