Chemists at the Indian Institute of Technology (IIT) in Chennai have shown that carbon nanotubes that contain nitrogen can potentially be used for storing hydrogen gas1.
The finding is significant with hydrogen emerging as a future energy source and scientists worldwide looking for a convenient medium to store it.
Hydrogen storage by carbon materials has been studied as a possible option earlier but pure carbon surface was not found suitable. The carbon surface must be modified by addition of metal or metal oxides that act as activation centers to dissociate or split the hydrogen molecule into atoms.
The researchers B. Viswanathan (left) and M. Sankaran. 
IIT chemists Sankaran Murugesan and Balasubramanian Viswanathan sought to find a way out. Two years ago they had theoretically shown that atoms of nitrogen, phosphorous, sulphur and boron -- collectively known as heteroatoms -- may be able to act as centers for hydrogen activation, and that the activated hydrogen may be able to migrate to the carbon surface more easily1. They hit upon this idea after realizing that addition of one or two heteroatoms to the carbon nanotubes significantly decreased the energy needed for dissociating the hydrogen molecule.
The scientists now claim to have obtained experimental proof2. They prepared carbon nanotubes containing nitrogen (NCNTs) and pure carbon nanotubes (CNTs) without nitrogen and compared their hydrogen storage capacity by a technique called 'evolved gas analysis.' They found that NCNTs could absorb 4 to 6 times more hydrogen than CNTs.
"It is very impressive to see significant enhancement in hydrogen uptake with nitrogen inclusion in the carbon network," says Kuei-Hsien Chen, who works in this area at the Institute of Atomic and Molecular Sciences of Taiwan's Science Academy. "The inclusion of nitrogen and boron in CNTs opens up new avenue for hydrogen storage," he said in an email interview.
"Our study shows that nitrogen-containing carbon nano-materials are amenable to hydrogen absorption compared to pure carbon materials," Viswanathan told Nature India. "Results confirm that nitrogen atoms in carbon matrices act as the active centers to activate gaseous molecular hydrogen and its subsequent transport to the carbon surface by 'spill-over' process." The IIT studies also showed that the nitrogen atoms should be geometrically and chemically stabilized in the carbon nanotube framework for reproducible hydrogen uptake.
The IIT scientists admit that their work is exploratory in nature and further studies are needed to substantiate the feasibility. Most importantly, the stability of the materials has to be established by storing and releasing hydrogen at least 5000 times. The IIT team has established the stability for only about 10 cycles. Secondly, the storage capacity of the nanotubes must be further increased by refining the surface engineering techniques. "It must go up by at least two times for the nanotubes to be of any use," Viswanathan said.
Although the scientists made and tested only carbon nanotubes containing nitrogen and boron, Viswanathan says, they have theoretically shown that sulphur and phosphorous can also be incorporated into the carbon nanotubes for hydrogen storage.
