Dec 9, 2009
Nanoconfined hydrides measured for hydrogen storage
An efficient way to store hydrogen still presents a major challenge for the introduction of a hydrogen-based economy. Hydrogen storage in solid materials is considered among the most attractive methods. The synthesis of nanosized hydrides gives developers new possibilities in this area and small-angle neutron scattering (SANS) is a unique technique to investigate the effectiveness of such nanoconfinement.
A system satisfying the targets set by the automobile industry should contain materials that meet the required properties with respect to hydrogen capacity, thermodynamics and kinetics for hydrogen uptake and release. In the field of H-storage in metal hydrides, the rationale behind "nanoengineering" is based not only on kinetic enhancement, but also on the fact that the increase in surface energy (when the particle size is reduced) may result in a decrease in the enthalpy of formation.
In a recent study, which was published in the journal Nanotechnology, researchers presented the first direct measurement of the successful nanoscale infiltration of hydrides in a carbon scaffold by using a combination of techniques, amongst them SANS.
Magnesium-borohydride particles, an attractive option thanks to the material's high theoretical hydrogen content, were incorporated in porous carbon scaffolds by using a wet impregnation procedure. The decomposition temperature of the nanocomposite system was lowered compared with the bulk hydride.
A complete picture of the nanoporous network of the scaffold materials is necessary to understand the kinetic improvement and to optimize the infiltration processes employed. The SANS measurements are crucial to study particle size, pore size and particle size distributions, due to the absence of defined diffraction peaks in the systems studied in this work.
Using the SANS data, the researchers have been able to estimate that the infiltrated particles have a significant component in the correlation length of around 4 nm. The reduced porosity of the system after infiltration suggests that the complex hydride particles are well integrated into the pores of the scaffold. It was not excluded that some material could be deposited on top of the grain surfaces, but the absence of diffraction peaks showed that this can consist of only very small particles.
One of the most important questions is how the hydride material is organized spatially within the pores of the scaffold, which has important implications for hydrogen absorption and desorption. The structural data obtained from the measured samples will be used for theoretical modelling to find the optimal size for improved thermodynamics and kinetics in our selected hydrides.
About the author
The work was supported by the EU project NANOHy. The SANS measurements were performed using the JEEP II reactor at the Institute for Energy Technology (IFE). Sabrina Sartori is a postdoctoral researcher in the Hydrogen Storage Group, which is part of the physics department at IFE. Kenneth D Knudsen is a researcher at IFE and adjunct professor at NTNU, Trondheim. Zhirong Zhao-Karger and Elisa Gil Bardaij are postdoctoral researchers at Karlsruhe Institute of Technology (KIT). Maximilian Fichtner is group leader at KIT. Bjørn C Hauback is group leader at IFE and adjunct professor at the University of Oslo.