Hydrogen kinetics in magnesium hydride: on different catalytic effects of niobium

The hydrogen desorption kinetics from pure and Nb-doped MgH2 samples was studied as function on the Nb concentration (6x10-4<Nb/Mg<5x10-2). Structural and kinetics analyses indicate that Nb acts as catalyst both when Nb atoms aggregate forming NbH clusters dispersed in the MgH2 and also when Nb is contained as atomic impurity. It is suggested that the local atomic environment around the Nb atom acts as seed for h-Mg phase nucleation which constitutes the rate limiting step for the hydrogen kinetics while, when the Nb concentration exceeds about 1 at.%, the rate limiting step is hydrogen diffusion.