[ materials ]
( BET ) N 2 surface area – a standard procedure for comparing surface areas . 10
Fig . 4 . Hydrogen adsorbed ( wt %) for different materials at saturation at 77 ° K ( -196 ° C ) versus the BET surface area ( m 2 g -1 ) 10
where Q ( kJ ) is the heat released during the reaction . The reaction that occurs during the storage of hydrogen is exothermic ( Q > 0 ), whereas during the release of hydrogen from the material it is endothermic ( Q < 0 ). The energy levels of solid-state hydrogen storage are schematically illustrated in Figure 3 , where it is seen that only chemisorbed hydrogen penetrates the metal , and the energy in kJ / mol H can be > 0 or < 0 , depending on storage versus retrieval , respectively .
Material categories The most common materials used for solid-state hydrogen storage are as follows :
• Metal-organic frameworks ( MOFs )
• Carbon-based materials :
• Carbon nanotubes ( CNTs )
• Graphene
• Fullerenes
• Activated carbons ( ACs )
• Zeolites
• Covalent organic frameworks ( COFs )
• Metal hydrides
Figure 4 compares the adsorption behaviour of several of these materials at saturation at 77 ° K ( -196 ° C ) versus the Brunnauer – Emmett – Teller
Metal-organic frameworks ( MOFs ) MOFs are a class of synthetic porous materials which have a microporous crystalline structure comprising of metal ions or clusters that are connected via molecular bridges . MOFs have good stability , high void volumes , well-defined tailorable cavities of uniform size , high surface areas , and adjustable pore sizes . The design flexibility for tuning the porosity of MOFs has attracted attention for their usage as hydrogen adsorbents .
Once the hydrogen is stored , pressure will be applied on the material ( through tank roof movement ), which will allow the hydrogen gas to be extracted . The concept is like a sponge with water – once put in water the sponge will store water therein , but once the sponge is squeezed the water will be removed . 5
Carbon-based materials Carbon-based nanomaterials have received considerable attention as potential hydrogen storage materials because of their low costs , low weights , high surface areas , good chemical stabilities , and wide diversities of bulk and pore structures . In particular , their use is advantageous in many industrial applications because of their moisture-resistant properties . There are many ‘ submaterials ’ in this category , including carbon nanotubes ( CNTs ), fullerenes , graphites , graphene derivates , and activated carbons ( ACs ). Among these materials , ACs present very promising candidates for hydrogen storage because of their high specific surface areas , chemical and mechanical stabilities , microporous structures , and low costs . Another advantage of ACs is that agricultural waste , such as coffee bean dregs , coconut husks , and rice husks , have been used as raw materials for manufacturing ACs , which helps reduce any adverse environmental footprint of their production . 9 , 12
Carbon nanostructures such as CNTs and nanofibers are being heavily researched lately .
Hydrogen Tech World | Issue 8 | February 2023 41