Enhanced hydrogen sorption properties of Ni and Co-catalyzed MgH2

MgH2 is one of the most promising hydrogen storage materials due to its high capacity and low cost. In an effort to develop MgH2 with a low dehydriding temperature and fast sorption kinetics, doping MgH2 with NiCl2 and CoCl2 has been investigated in this paper. Both the dehydrogenation temperature and the absorption/desorption kinetics have been improved by adding either NiCl2 or CoCl2, and a significant enhancement was obtained in the case of the NiCl2 doped sample. For example, a hydrogen absorption capacity of 5.17 wt% was reached at 300 °C in 60 s for the MgH2/NiCl2 sample. In contrast, the ball-milled MgH2 just absorbed 3.51 wt% hydrogen at 300 °C in 400 s. An activation energy of 102.6 kJ/mol for the MgH2/NiCl2 sample has been obtained from the desorption data, 18.7 kJ/mol and 55.9 kJ/mol smaller than those of the MgH2/CoCl2, which also exhibits an enhanced kinetics, and of the pure MgH2 sample, respectively. In addition, the enhanced kinetics was observed to persist even after 9 cycles in the case of the NiCl2 doped MgH2 sample. Further kinetic investigation indicated that the hydrogen desorption from the milled MgH2 is controlled by a slow, random nucleation and growth process, which is transformed into two-dimensional growth after NiCl2 or CoCl2 doping, suggesting that the additives reduced the barrier and lowered the driving forces for nucleation.