"Hydrogen storage infrastructures have become one of the major obstacles in the development of the global hydrogen energy economy," researcher Zahir Dehouche told nanotechweb.org. "Currently known storage materials do not reversibly release hydrogen within the temperature range of operation of PEM [proton exchange membrane] fuel cells and do not have the capacity, charge/discharge rate, or life cycle to allow fuel cells to be a competitive alternative."

Dehouche and colleagues used mechanical grinding to prepare titanium and zirconium-doped sodium aluminium hydride (NaAlH4) composites with carbon additives. They employed carbon in three forms - single-walled carbon nanotubes, graphite or activated carbon. The single-walled carbon nanotubes were made by the high pressure CO conversion (HiPco) process, activated carbon or graphite

"During the last years alkali-metal aluminium hydrides have been intensely examined for reversible hydrogen storage applications, but there are many problems associated with their use, including unstable reversible thermodynamic properties and unfavourable kinetics," said Dehouche. "An efficient approach, which apparently has not been studied much, appears to be the formulation of new carbon/transition metal catalyst composites of specific composition and molecular structure, which can greatly stimulate and improve the chemical reactions involving hydrogen relocation in alkali-metal aluminium materials."

The researchers tested the hydrogen absorption and desorption properties of the composites under pure hydrogen at 160° C for up to 200 cycles. They used a hydrogen absorption pressure of 35 atm and a desorption pressure of 0.25 atm.

The hydriding and dehydriding kinetics of single-walled nanotube/catalyzed sodium aluminium composite were four times better than those of the material ground without carbon additives. The hydrides ground with graphite and activated carbon additives also had faster desorption and absorption than the plain materials, but to a lesser extent.

The researchers believe that the presence of carbon creates new hydrogen transition sites and it appears that the structure of the carbon is important. The scientists reckon that the high hydrogen diffusivity of the nanotubes facilitates hydrogen atom transition. Faster thermal energy transfer through the nanotubes may also help reduce hydriding and dehydriding times.

However, after 200 cycles the single-walled nanotube/catalyzed sodium aluminium composite lost some of its storage capacity.

Now the team says it wants to investigate the effect of carbon on high pressure (120 atm) hydriding kinetics; understand the mechanism by which the carbon materials enhance the hydrogen sorption kinetics of catalyzed sodium aluminium hydride and why the superior performance of single-walled nanotubes over graphite and activated carbon is not conserved after 200 cycles; and apply the same doping technique to alkali-metal aluminium hydrides based on magnesium and lithium in order to have greater potential storage capacity.

The researchers reported their work in Nanotechnology.