Hydrogen absorption and desorption kinetics of Ag-Mg-Ni alloys

The hydrogen absorption/desorption (A/D) kinetics of hydrogen storage alloys Mg2-xAgxNi (x = 0.05, 0.1) prepared by hydriding combustion synthesis in two-phase (alpha-beta) region in the temperature range of 523-573 K have been investigated. The hydriding/dehydriding (H/D) reaction rate constants were extracted from the time-dependent A/D curves. The obtained hydrogen A/D kinetic curves were fitted using various rate equations to reveal the mechanism of the H/D processes. The relationships of rate constant with temperature were established. It was found that the three-dimensional diffusion process dominates the hydrogen A/D. The apparent activation energies of 63 +/- 5 and 61 +/- 7 kJ/mol H-2 in Mg1.95Ag0.05Ni alloy and 52 +/- 2 kJ/mol H-2 and of 50 +/- 2 kJ/mol H-2 in Mg1.9Ag0.1Ni alloy were found for the H/D processes in two-phase (alpha-beta) coexistence region from 523 to 573 K, respectively. With the increasing content of Ag in Ag-Mg-Ni alloys, the apparent energy was decreased and the reaction rate was faster. It is reasonable to explain that the hydriding kinetics of Mg2Ni was improved by adding Ag. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.