Cycling and thermal stability of nanostructured MgH 2–Cr 2O 3 composite for hydrogen storage

Abstract

This paper presents investigations on the long-term cycling stability as well as the thermal stability of the hydriding–dehydriding properties of nanostructured MgH 2 with 0.2 mol.% Cr 2O 3 catalyst additions synthesized by ball milling. The hydrogen charge and discharge stability of the nanocomposite hydride has been tested at 300 °C 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 nm 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 °C, while exposure to 400 °C over 55 h leads to coarsening of the crystallite size and concomitant loss of the excellent kinetic properties.