Morphology evolution of bimetallic Ni/Zn-MOFs and derived Ni3ZnC0.7/Ni/ZnO used to destabilize MgH2

Herein, 3D porous flower-like Ni/Zn-MOFs are synthesized via a simple solvothermal method. The reaction time shows obvious effects on the morphology evolution of Ni/Zn-MOFs, and the morphology evolution from plate to 3D flower is governed by a “nucleation–dissolution–renucleation” mechanism. Nonstoichiometric bimetallic carbide-containing composites (Ni3ZnC0.7/Ni/ZnO composites) are obtained through the simple calcination of Ni/Zn-MOFs under 4 MPa H2, suggesting that ZnO cannot be reduced and Ni3ZnC0.7 is stable even under 24 h of hydrogen treatment. The MgH2-5 wt% Ni3ZnC0.7/Ni/ZnO-3 composite began to dehydrogenate at 230 °C and 5 wt% H2 can still be released within the initial 1 h at 573 K. The hydrogen absorption capacity of the composite reached 2.57 wt% H2 at 423 K within 600 s. Interestingly, the composite can still absorb 0.78 wt% H2 at 353 K within 60 min. Compared to pure MgH2, the activation energy (Ea) of hydrogen absorption and dehydrogenation of the composite decreased to 44.85 and 93.06 KJ/mol, respectively. Comparing the cases of Ni3ZnC0.7, Ni, and ZnO addition clearly reveals that Ni3ZnC0.7 and Ni are the primary catalysts in these improvements. The Ni3ZnC0.7/Ni/ZnO-3 composite will react with MgH2 during the absorption–desorption cycle, and the in situ generated Mg2Ni/Mg2NiH4 acts as a “hydrogen pump” to reduce the potential barrier for the dissociation and recombination of H2. Trace amounts of MgZn2 can promote the formation of Mg/MgH2 and the resulting minor expansion and contraction during phase changes provides more paths for hydrogen diffusion. This study clearly demonstrates the efficient catalytic activity of nonstoichiometric bimetallic carbides and provides a promising catalyst design for Mg-based hydrogen storage materials.