Hydrogen desorption kinetics of a mechanically milled MgH 2+5at.%V nanocomposite

Abstract

The hydrogen desorption kinetics of mechanically milled MgH 2+5at.%V nanocomposite were determined under various desorption pressures and temperatures. The reaction rate constant was extracted from the time-dependent desorption curves. The relationships of rate constant with pressure and temperature were established. It was found that the hydrogen desorption at high temperature and under high driving force, is controlled by the interface (Mg/MgH 2) motion. When the driving force is small, the early stage of hydrogen desorption is controlled by nucleation and growth and the later stage is controlled by long range hydrogen diffusion. At temperatures below 523 K, the nucleation and growth process dominates the hydrogen desorption. High temperature annealing (673 K) of the nanocomposite results in slower desorption kinetics and increased activation energy of desorption. At high temperatures, the rate-limiting step changes from interface control (before annealing) to surface control (after annealing), while at low temperatures, the rate-limiting step of desorption does not change after annealing.