# Reversible Hydrogen Storage Using Nanocomposites
## 2 Nanocomposite Materials for Hydrogen Storage
### Polymeric Nanostructures  
Polymer nanocomposite materials consist primarily of polyaniline, a conducting polymer, which has conductivity on the order of 1 Siemens/cm [79]. The chemical makeup of polyaniline in its emeraldine form has anions, which allow hydrogen ions to bond to the material very well. Due to its porosity, the surface area of the conducting polymeric matrix material will allow for additional hydrogen binding in the form of physisorption. The polymer nanostructures are widely studied for their interesting physical and chemical properties [80; 81; 82; 83] and are deployed for practical applications [84; 85; 86]. Conducting polyaniline is a type of polymer that could be prepared in powder, gel, or other suitable forms and have employed them for storing hydrogen at varying densities [87; 88]. The physical adsorption mechanism of hydrogen both on the surface and into the bulk structure of the polyaniline is widely studied by several researchers [89; 90; 91; 92]. Similar to carbon nanotubular [75; 93] and nanofibrillar [94] structures, polyaniline could be prepared in nanoforms for increasing their surface area for maximizing the hydrogen adsorption [95] and other physicochemical applications such as chemical/biochemical sensors [96], actuators [97], and high speed memory devices [98].  
In the chemical oxidative polymer condensation process of aniline, one can easily produce the fibrous nanostructures that could form with a few nanometers thickness and a micrometer length. These nanofibers are in the real sense grown on the granular particles of polyaniline solution. There may be a number of preparation techniques such as interfacial polymerization and chemical templating that can readily generate the polyaniline nanofibers that are of great control structures, and process mechanisms. Based on this highly controlled template synthesis process, various polyaniline nanostructures have been produced in recent times, [99] and their structural morphology and other properties have been optimized [100]. The electrospinning is another useful and potential technique by which we have prepared our polyaniline nanofibers [101; 102] whose morphology is shown in Figure 14. The reversible hydrogen uptake at room temperature was performed on these polyaniline nanofibers and is demonstrated in Figure 15. Since the molecular hydrogen binding energies need to be adjusted by understanding the mechanistic behavior of these polyaniline nanofibers, the reversible hydrogen storage characteristics is an ongoing research and development.  
Doping with carbon nanotubes and fullerenes, as well as catalyst materials enable the formation of chemical and physical bonds at the materials interface. By combining different carbon nanovariant  
Figure 14: SEM images showing the surface morphology of electrospun polyaniline nanofibers.  
Figure 15: Hydrogen sorption cycle-life of electrospun polyaniline nanofiber at 100 \({}^{\circ}\)C. A1–A4: Absorption in four consecutive run and D1–D3: desorption after every hydrogen absorption for three consecutive runs.  
structures, a new hydrogen storage system can be manufactured that exceeds the performance of the existing materials and facilitates commercialization of hydrogen storage systems. These new catalyst doped hydrogen storage materials can be described as porous nanocomposites of a conducting polymer with modified fullerene compounds and carbon nanotubes, as well as kinetics enhancing catalyst materials. The combination of the high surface area of carbon nanotubes and modified fullerenes [103] with the tunable redox behavior of the conducting polymer potentially offers enhanced performance as a reversible hydrogen storage system.