Electrochemical storage of hydrogen using carbon nanotubes

Abstract

Hydrogen has always been an attractive fuel with only one byproduct produced during combustion. Presently, there isn’t any material capable of handling the storage of hydrogen. It has been claimed that metal hydrides are the most effective way of storing hydrogen but this is still not enough to meet the US Department of Energy of 6.5 wt % - the amount of energy sufficient to power a fuel cell car for a distance of 500 kilometers. Research done with respect to hydrogen storage using carbon nanotubes had shown some potential and Chen et al. (Chen 1999) had claimed that a 20 wt % of hydrogen can be obtained by gas intercalation using alkaline metal doped multi-walled nanotubes. However, this finding was soon refuted by Pinkerton who next proved that most of the storage is water and not hydrogen. (Pinkerton 2000) Another form of research being conducted is hydrogen storage using electrochemical cell. The storage capacities obtained in this research are still rather low but this research is still in its infancy stage and much would be expected only in the near future.For this thesis, catalysts were made using impregnation and co-precipitation and some were verified using the TEM images. The images showed that the carbon nanotubes were multi-walled nanotubes. These multi-walled nanotubes were purified by acid oxidation for the removal of metal catalysts and support. These multi-walled nanotubes were then used for the electrochemical storage of hydrogen. Different methods of treatment for the multi-walled nanotubes had been adopted and it is realized that hydrogen storage capacity of greater than 1.77 wt% is obtained from ball milled multi-walled nanotubes and LaNi5. LaNi5 itself can be used for electrochemical storage of hydrogen. By adding LaNi5 to the multiwalled nanotube sample, it is believed to have decorated the walls of the multiwalled nanotubes thus, allowing easier diffusion of hydrogen into the carbon nanotubes. Another astonishing result achieved, is by subjecting the purified multi-walled nanotubes to hydrogen treatment at 900OC for an hour and cooling it down to room temperature with argon gas. This treatment was initially meant for the removal of functional groups on the tubes, but a massive storage of 0.92 wt% was observed. It was suspected that spillover of atomic hydrogen from the catalyst to the carbon nanotubes may have occurred. The accumulated hydrogen which was not desorbed during cooling, were later discharged in the electrochemical cell test.