Materials for hydrogen storage can be divided into two families: hydrides (hydriding alloys, molecular hydride complexes, amine complexes, and hydrocarbons) and physisorbed high-surface-area materials (carbon fullerenes, nano-tubes, metal-organic frameworks and aerogels).
Materials for hydrogen storage can be divided into two families: hydrides (atomic hydrogen resides mainly in the bulk of the material), this includes hydriding alloys, molecular hydride complexes, and other molecular covalent compounds such as amine complexes, and hydrocarbons; and physisorbed high-surface-area materials (hydrogen resides mainly as molecular hydrogen on the surface of the material), this includes carbon fullerenes, nano-tubes and highly porous media like metal-organic frameworks and aerogels.
Nano-structured materials have much higher surface area to volume ratios than bulk materials, enabling increased adsorption. Nano-structuring of materials also improves reaction kinetics by increasing the diffusivity, reducing the reaction distance and increasing the reaction surface area. Metal hydrides and hydride complexes can be nano-structured by a variety of different processes including sputtering, pulsed laser deposition, and mechanical milling.
Nano-structured carbons, such as nanotubes, fullerenes and graphitic sheets are examples of nano-structured materials that have been extensively studied for hydrogen storage.
Porous materials are being studied for use as hydrogen storage media due to their high surface area to volume ratio and the ability of hydrogen to adsorb to these internal and external surfaces. Moreover, for microporous materials, (pore width <2 nm), attractive physical potentials from opposite walls can overlap leading to a fully active space for the gas to adsorb. Therefore, such nanoscale pores can offer interesting gains over compression comparatively to macro pores, where such overlap is virtually absent and gas is stored mainly under a compressed state into the larger voids. Although adsorption via physisorption is significant for porous media, absorption may also contribute to the overall hydrogen storage capacity of advanced materials. Examples of porous materials being investigated for hydrogen storage are various forms of aerogels, clathrates, carbon-based materials and metal-organic frameworks (MOFs). The advantage of using an adsorbent over simple compression is temperature and pressure dependent.
