Physicists in the US may have found a promising new class of material that can absorb and store large amounts of hydrogen. Adam Phillips and Bellave Shivaram of the University of Virginia measured around 12% by weight of hydrogen uptake in the metal-based composites. This is significantly higher than the target of 5.4wt% set by the US Department of Energy to support the development of hydrogen-powered vehicles — although the physicists say much work still needs to be done.

A low-cost, high-capacity hydrogen-storage medium is essential for the commercialization of hydrogen fuel-cell technologies in the future. While scientists have investigated various classes of material, such as carbon nanotubes, hydrogen-clathrate-hydrates and other nanostructured materials over the past few decades, no one satisfactory material has yet been found.

Transition metal-ethylene complexes

Now, Phillips and Shivaram report on transition metal-ethylene complexes with promising hydrogen-storage properties (Phys. Rev. Lett. 100 105505). "Several theory papers have suggested that if you isolate a titanium (Ti) atom with a carbon nanostructure, the Ti is capable of weakly bonding with three to five hydrogen molecules," explained Phillips. "Our experiments focused on a Ti-ethylene structure predicted to bond up to 12wt% for 1:1 Ti:ethylene or 14wt% 2:1 Ti:ethylene — in agreement with our results."

Phillips and Shivaram obtained their results by first vaporizing Ti atoms in an ethylene atmosphere. The Ti atoms are thought to bond with the ethylene before being deposited on surface acoustic wave (SAW) mass sensors. Once the deposition is complete, the researchers evacuate the excess ethylene from the chamber and introduce one atmosphere of hydrogen.

It is critical to say that our work is at a very early stage Adam Phillips, University of Virginia

Throughout the process, the scientists measure the mass of hydrogen accumulating on the sensors using a nanogravimetry technique. Here the resonant frequency of the SAW device decreases with increasing mass, so the precise amount of hydrogen loading onto the Ti-ethylene complexes can be determined by simply measuring this frequency.

'Goldilocks' regime

"We believe that isolated transition metals (as we think we have) can bond to hydrogen molecules in a 'Goldilocks' regime — stronger than physisorbtion but weaker than chemisorbtion," said Phillips. This is an advantage because most physisorbtion materials absorb hydrogen only at very low temperatures. In contrast, chemisorbtion materials dissociate the hydrogen molecule during absorption, which means that the materials form strong bonds with hydrogen that require elevated temperatures.

The researchers now plan to scale up the nanogram quantities of materials that they have studied. They also hope to investigate the bonding mechanism by vaporizing Ti in gases, such as benzene and other cyclic organic compounds.

"It is critical to say that our work is at a very early stage," added Phillips. "While we have measured the hydrogen uptake, we have not yet been able to determine how the material desorbs. However, we are very optimistic about the possibility of scaling up and overcoming many of the other hurdles we now face."