Stabilization of low-valence transition metal towards advanced catalytic effects on the hydrogen storage performance of magnesium hydride

Magnesium hydride(MgH_(2))has been widely regarded as a potential hydrogen storage material owing to its high gravimetric and volumetric capacity.Its sluggish kinetics and high activation energy barrier,however,severely limit its practical application.Transition metal oxides(TMOs)have been extensively used as catalysts to improve the hydrogen storage performance of MgH_(2),but the low-valence transition metal(TM)ions,resulting from the reduction of TMOs accompanied by the formation of inactive Mg O,have been demonstrated to be the most effective components.Herein,we theoretically and experimentally confirm that the doping of low-valence TMs into Mg O could effectively weaken the Mg-H bonds and decrease the energy required for hydrogen desorption from MgH_(2),leading to superior catalytic activity compared to both TMOs and Mg O.In particular,the apparent activation energy for the dehydrogenation of Mg(Nb)O-catalyzed MgH_(2)could be reduced to only 84.1 kJ mol^(-1),and the reversible capacity could reach around 7 wt.%after 5 cycles with a capacity retention of 96%.Detailed theoretical calculations confirm that the remarkable orbital hybridization between Mg(Nb)O and MgH_(2)promotes charge transfer from MgO to the MgH_(2)monomer,resulting in significantly weakened stability of MgH_(2),which could effectively enhance its hydrogen storage performance.

Effects of heat treatment on mechanical properties of an extruded Mg-4.3Gd-3.2Y-1.2Zn-0.5Zr alloy and establishment of its Hall–Petch relation

The effects of T4,T5,and T6 treatment on the microstructure and mechanical properties of the extruded Mg-4.3Gd-3.2Y-1.2Zn-0.5Zr(wt.%)alloy with a relatively low RE content(7.5 wt.%)were investigated.T4 treatment at 450–500°C induces a gradual grain growth ofα-Mg but an obvious transition of texture component from<0001>⊥ED to<0001>∥ED.Interdendritic LPSO phases are highly stable against annealing while intragranular ones experience dissolution and re-precipitation.After peak-ageing at 200°C,the elongation of as-extruded and T4 samples is just slightly reduced or even increased due to the weak ageing hardening response.T5 sample exhibits an attractive combination of strength and ductility,with a tensile yield strength(TYS)of 303 MPa and elongation of 20.0%.The Hall–Petch relation for the alloys with or without ageing treatment has been estimated.Grain boundary strengthening rather than precipitation strengthening has the dominant contribution to TYS,and a modified equation is developed to predict grain boundary strengthening values for Mg-Gd-Y-Zn-Zr alloys which contain different Schmid factors for basal slip.