Enhanced “Hydrogen Pump” Effect in Improving the Hydrogen Storage Performance of MgH ₂ via the Introduction of a Flower-like NiCu Bimetallic MOF

To overcome the hydrogen storage kinetic limitations of magnesium hydride (MgH₂), highly effective NiCu bimetallic MOFs with different Ni/Cu ratios were successfully synthesized and introduced into MgH₂. The Ni₃Cu₁ MOF, featuring a distinctive flower-like structure assembled from thin nanosheets, demonstrates the most remarkable catalytic activity in enhancing the hydrogen storage properties of MgH₂. The MgH₂-8 wt % Ni₃Cu₁ MOF composite can initiate hydrogen desorption at 198.50 °C and exhibits a significantly reduced dehydrogenation peak temperature of 285.92 °C, in comparison with that of the ball-milled MgH₂ (385.5 °C). This composite can rapidly release 6.18 wt % H₂ at 300 °C and absorb 5.32 wt % H₂ at 150 °C, both within 500 s. The activation energies for dehydrogenation and hydrogenation are reduced to 66.91 and 33.81 kJ/mol H₂, respectively. Furthermore, the MgH₂-8 wt % Ni₃Cu₁ MOF presents a capacity retention rate of 93% after 50 cycles, indicating excellent cycle stability. The enhancement in the hydrogen storage performance is mainly attributed to the following two aspects. The distinctive flower-like Ni₃Cu₁ MOF possesses a relatively large specific surface area, which provides more uniformly dispersed metal active sites and a larger contact area with MgH₂. Moreover, the Mg₂Ni(Cu)/Mg₂Ni(Cu)H₄ phases formed in situ during the initial hydrogen de/absorption cycle act as an enhanced "hydrogen pump" to facilitate the de/hydrogenation process of MgH₂/Mg. The enhanced effect can be confirmed by theoretical calculations, which reveal a pronounced interfacial electron transfer (0.54 e⁻) and a marked elongation of the Mg-H bond (2.85 Å) at the MgH₂-Mg₂Ni(Cu) interface compared to that of the MgH₂-Mg₂Ni interface. These findings offer a strategy for the design and preparation of bimetallic MOF catalysts to improve the hydrogen storage properties of Mg-based materials.