Synthesis and hydrogen absorption/desorption properties of Mg-Nb₂O₅-SWCNT/MWCNT nanocomposite prepared by reactive milling

Mg-Nb₂O₅-SWCNT and Mg-Nb₂O₅-MWCNT composites were prepared by ball-milling in a hydrogen reactive atmosphere (P = 3.0 MPa) for different periods of time, and their sorption behaviors and microstructure were investigated systematically. The results showed that the hydriding and dehydriding kinetics of Mg-Nb₂O₅-SWCNT and Mg-Nb₂O₅-MWCNT 1 h milled samples were markedly improved. At 573 K, these two composites could absorb 5.81-4.88 wt% hydrogen under initial hydrogen pressure of 3.0 MPa, and desorb 5.14-4.01 wt% hydrogen within 600 s. Furthermore, these 1 h milled composites could also absorb hydrogen in a moderate temperature range between 298 K and 573 K. However, the Mg-Nb₂O₅-SWCNT 1 h milled sample had better hydriding/dehydriding kinetics than the Mg-Nb₂O₅-MWCNT 1 h milled sample. The apparent activation energies for the hydrogen absorption and desorption of the Mg-Nb₂O₅-SWCNT 1 h milled sample were estimated to be 12.84 kJ/mol and 102.69 kJ/mol from the Kissinger's plot. These figures were lower than those of a pure Mg sample with the same milling time, which were 3.14 and 2.0 times higher, respectively. The desorption temperature and the dehydrogenation enthalpy of the Mg-Nb₂O₅-SWCNT 1 h milled sample were 140 K and 20.72 kJ/mol, respectively, which were lower than those of pure Mg/MgH₂.