[ materials ]
Covalent organic frameworks ( COFs ) COFs are organic structures which are bonded by tough covalent bonds ( e . g ., C-C , C-O , or Si-C ) instead of metal ions to allow formation of highly porous , highly effective surface area materials with low crystal density . COFs are structured to contain pore dimensions comparable in length to the scale of a hydrogen molecule ’ s diameter . 5 Their lightweight properties allow their deployment practically and easily .
Fig . 5 . Volumetric and gravimetric hydrogen storage densities of different metal and complex hydrides 15
They have cylindrical nanostructures generated by rolls of graphene . Their diameters can range from 0.7 nm to various nanometres and form single and multi-walled tubes that form close-packed bundles , with lengths of 10 – 100 micrometers ( μm ). 13 There have been instances proving stable hydrogen storage at room temperature in carbon nanostructures , with capacities < 10 wt % in singlewall nanotubes ( SWNT ). 12
Graphene is generated from graphite . Graphene has a porous structure with superior stabilized chemical properties and reversibility . Such a framework enables hydrogen absorption and desorption with good affinity . Graphene is considered more coherent than CNTs since they are safer and operationally easier to prepare . 5
Zeolites Zeolites are microporous aluminosilicates formed from AlO 4 and SiO 4 tetrahedra . 13 Zeolites have wide applications especially as catalysts in hydrocarbon refining . Their distinctive ion exchange , molecular sieving , and catalytic properties allow their use in a wide range of operating parameters . Hydrogen is captured in zeolites cavities , but once a prescribed temperature is applied , the hydrogen can be released . Zeolites exhibit considerable thermal stability , and their compositions are easily adjustable . 5
Metal hydrides Storage of hydrogen in metal hydride materials involves the dissociation of molecular hydrogen into atomic hydrogen at the surface of the material and then the penetration or ingress into the bulk material , interstitially . Many metallic compounds exist that absorb hydrogen in this method . It is worth pointing out that in practical applications the absorption does not take place at convenient ( e . g ., ambient ) pressures and temperatures . Several interstitial hydrides possess affinity to hydrogen absorption and desorption properties , by means of pressure , temperature , kinetics , and resistance to gas impurities . Materials that fall into the interstitial hydride category include Pd and LaNi 5
, intermetallic compounds such as TiFe , and Vabased solid-solution alloys . 14
Figure 5 shows a plot of volumetric and gravimetric hydrogen storage densities of different metal and complex hydrides . 15 Metal hydrides are represented with squares and complex hydrides with triangles . BaReH 9 has the highest known hydrogen to metal ratio ( 4.5 ),
Mg 2
FeH 6 has the highest known volumetric density , and LiBH 4 has the highest gravimetric density . Reported values exclude tank weight , whereas the U . S . Department of Energy target lines include tank weight ; thus , the hydrogen storage characteristics of the shown hydrides are optimistic . It is worth noting that although hightemperature materials like Mg-based alloys can
42 Hydrogen Tech World | Issue 8 | February 2023