Michael J. Zehetbauer

Abstract:

There is the widespread opinion within the research community of hydrogen storage materials, that their nanocrystalline structure is a precondition for enhancement of the kinetics of hydrogen absorbtion/desorption, by means of enhanced diffusion of H2 along the grain boundaries. Examples for this behavior are presented, including ball milled, filed and/or SPD processed Mg, Mg- and Fe-alloys. SPD (Severe Plastic Deformation) represents a new method processing method to achieve bulk nanocrystalline materials. Considering more than one storage cycles in pure Mg, the kinetics and even the storage capacity is drastically decreased. One may find the reason in the strongly increased grain size because of the comparably high absorption/desorption temperature of 350°C. However, in the SPD processed Mg-alloys like ZK60, the kinetics and the storage capacity do not decrease with repeated absorption/desorption cycles, although the average grain size significantly increases. Thus it is concluded that the grain size effect beneficial to storage kinetics during the first cycles must have a reason other than hydrogen grain boundary diffusion. Recent DFT calculations within a research project of the authors suggest that the dissocation of H2 to H - which is a precondition of successful absorption of hydrogen by the host material - occurs more easily at the surface and especially in the wake of crystal defects being present at the surface. Experiments by the authors done in Mg-alloys and Fe-Ti having various initial ball milling particle sizes and/or grain sizes confirm this conclusion. Furthermore, recent experiments both from literature and from the authors showed that application of SPD can save the pulverization and/or filing of the H2 storage materials, by providing not only high densities of grain boundaries but also those of microcracks at the surface.