Synergistic effects of transition metal halides and activated carbon nanofibers on kinetics and reversibility of MgH2

MgH2 doped with transition metal halides (TiF4, NbF5, and ZrCl4) and activated carbon nanofibers (ACNF) for reversible hydrogen storage is prepared by ball milling technique. Transition metal halides provide catalytic effects for de/rehydrogenation kinetics, while ACNF benefits thermal conductivity and hydrogen permeability as well as prevents particle agglomeration during cycling. Significant reduction of onset and main dehydrogenation temperatures of MgH2 (ΔT = 243 and 158 °C, respectively) are achieved by doping with 5-10 wt % of NbF5, ACNF-TiF4 and ACNF-NbF5. During the 1st cycle, the latter samples liberate 4.7-5.0 wt % H2 within 1 h 30 min, whereas MgH2 doped with ACNF reaches only 1.5 wt % H2. The reaction between MgF2 and NbHx (x < 1) (MgH2-NbF5 and MgH2-ACNF-NbF5) during dehydrogenation results in the formation of new catalytic active species of Nb-F-Mg favoring kinetics. Upon four hydrogen release and uptake cycles, kinetics and reversibility within 1 h 30 min of MgH2-ACNF-NbF5 are preserved at 5.0 wt % H2, while those of MgH2-NbF5 and MgH2-ACNF-TiF4 decay to 4.4 wt % H2. Activation energy (EA) for dehydrogenation of MgH2 considerably decreases from 140.0 ± 10.2 to 37.8 ± 1.5 kJ/mol after doing with ACNF-NbF5. Superior performance of MgH2-ACNF-NbF5 to MgH2-NbF5 is due to synergistic effects of NbF5 and ACNF. In the case of MH2-ACNF-TiF4, the disappearance of active species benefiting kinetic properties and the formation of thermally stable TiH2 account for inferior hydrogen content reversible.