Global climatic patterns are changing drastically, among other factors contributing to this change are fossil fuel emissions. Rapid rise of earth’s surface temperature urges to move towards clean burning technologies, cleaner and renewable energy resources. Wind, hydropower, oceanic thermal energy and hydrogen energy are renewables. Among aforementioned resources hydrogen is reflected upon as eco-friendly energy source.
Hydrogen energy is an ideal concept in renewable resource situation, due to zero emissions and subsequent reduction in urban pollution. But to use hydrogen as an energy source its production and storage are two chief factors of vital importance. Between these two aspects storage of hydrogen is crucial. Prevailing approaches for hydrogen storage are inefficient, as hydrogen is stored either as a gas of liquefied under very low pressure. Gaseous hydrogen is physisorbed (weak interaction with the substrate) or chemisorbed (very strong binding forces are involved). Over the decades novel materials are being designed for proficient hydrogen storage including carbonaceous materials (activated carbon, carbon nanotubes and graphene materials) and mesoporous materials.
Carbonaceous materials are by far favored materials for Hydrogen storage. Carbon is a well-known adsorbent owing to its existence in amorphous form and its particular interactions within the material and with the adsorbate. Hydrogen is adsorbed near the surface of carbon materials through weaker interactions known as Van der Waals forces.
Lately carbon materials are designed so as to improve porosity to have better adherence of molecules. Microporous and ultra-microporous materials are synthesized having higher adsorption capacities. Adsorption of hydrogen over activated carbon (AC) is highly specific as it adsorbs on micropores, and to have appropriate micropore volume, AC is not sufficient for hydrogen storage.
Carbon’s tube structures are gaining success owing to higher adsorption rates and better mechanical strength. Carbon nanotubes (CNTs) are rolled sheets of graphene having diameter ranging from few nanometers to 100nm. They are categorized in terms of their wall structure, either singe walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs).
Although the storage capacity of CNTs is higher than AC however the pristine capacity is quite less, which can be boosted by functionalization of CNTs. Functionalization can be achieved by doping (incorporation of other metals). Doping of tubular material with different metals has proven to enhance the storage capacity by manifolds. Fullerenes a ball like carbon structure of sixty carbon atoms (C60) functionalized with Calcium stores more than 8.4 wt. % Hydrogen.
CNTs are promising candidate for hydrogen storage yet synthesis of significant amount of tubes is a task. Purity of synthesized material needs to be ensured to be used as storage material, but to obtain a pure material is a challenge. Most widely used method for singe walled carbon nanotubes (SWCNTs) is the arc-discharge method in a presence of a catalyst. But so formed tubes have other metallic components from catalyst thus compromising the purity. So as to purify SWCNTs numerous treatment technologies are used as acid treatment, oxidation or high temperature methods, which further alters the original properties of nanotube.
Researches over the recent decade have overcome the challenges of synthesis methods. Furthermore, functionalization of base material enhances adsorption capacity and consequently storage thus moving at greater pace towards hydrogen society.
By: Marria Ghalib