Cycling and thermal stability of nanostructured MgH2-Cr2O3 composite for hydrogen storage

This paper presents investigations on the long-term cycling stability as well as the thermal stability of the hydriding-dehydriding properties of nanostructured MgH2 with 0.2 mol.% Cr2O3 catalyst additions synthesized by ball milling. The hydrogen charge and discharge stability of the nanocomposite hydride has been tested at 300 degreesC for up to 1000 cycles. Between the first and the 500 or 1000 cycle, an increase of hydrogen storage capacity by about 8% is observed in dynamic and pcT measurements. This is attributed to structural relaxations and crystallite growth. While absorption kinetics remain almost as fast as in the first cycle, the kinetics curves show a significant and systematic slow-down of the desorption rates by about a factor of four. Evaluation of X-ray patterns before and after 1000 cycles reveals crystallite growth from initially 21 run to 84 nm after 1000 cycles. This microstructural coarsening during cycling may explain the decreasing desorption rate of the nanocrystalline magnesium based composite. Annealing experiments of the as-milled material show, that the microstructural coarsening is rather slow at temperatures up to 350 degreesC, while exposure to 400 degreesC over 55 h leads to coarsening of the crystallite size and concomitant loss of the excellent kinetic properties. (C) 2002 Elsevier Science B.V. All rights reserved.