Hydrogen Storage in Carbon Single-wall Nanotubes

Abstract

Carbon single-wall nanotubes (SWNTs) and other nanostructured carbon materials have attracted considerable interest recently due to several reports of high hydrogen storage capacities at room temperatures. Other reports indicate room temperature storage capacities are ~ 0 wt%. All in all, the reported capacities on these materials range from 0 to 60 wt%. Careful work at NREL indicates a maximum capacity for adsorption of hydrogen on SWNTs is ~8 wt%. Samples displaying this maximum value were prepared by sonicating purified SWNTs in a dilute nitric acid solution with a high-energy probe. The process cuts the SWNT into shorter segments and introduces a Ti-6Al-4V alloy due to the disintegration of the ultrasonic probe. The Ti-6Al-4V alloy is a well-known metal hydride and its contribution to the measured hydrogen uptake must be accounted for in order to assess the amount of hydrogen stored on the SWNT fraction. One way to evaluate the relative importance of the cutting and the presence of the alloy in producing active SWNT materials is to produce cut samples that do not contain metal impurities. Metal hydrides may then be subsequently introduced in a controlled manner. This paper describes the experiments used to evaluate the hydrogen storage capacity of the sonicated SWNTs, and introduces new methods for cutting SWNTs that avoid the introduction of the impurity alloy. A Raman spectroscopy technique was also developed which enables the degree of cutting to be tracked. This year we also developed a method for growing SWNTs by chemical vapor deposition from methane that promises, when scaled-up, to produce SWNTs for approximately $1/kg. This latter work will not be discussed here, but will soon be submitted to the peer-reviewed literature.