Synchronously upgrading of hydrogen storage thermodynamic, kinetics and cycling properties of MgH2 via VTiMn catalyst

To effectively enhance the hydrogen storage performance of MgH2, the multimetallic catalyst VTiMn was triumphantly synthesized by mechanical alloying, and the MgH2-10 wt% VTiMn composite was further fabricated. The results revealed that VTiMn significantly improved the thermodynamic, kinetics and cycle properties of MgH2. Compared with pure MgH2, VTiMn promotes its plateau pressure to increase by 0.16 MPa and the initial desorption temperature to decrease by 47 K, showing obvious thermodynamic instability. Meanwhile, the MgH2-VTiMn composite can adsorb 5.5 wt% H-2 within 72 s and release within 1200 s at 523 K. The activation energies of hydrogen absorption and desorption are 30.1 and 90.5 kJ/mol, respectively. After 130 cycles at 598 K, 83 % hydrogen capacity retention can still be achieved. Characterisation indicated that the composite formed a core-shell structure with MgH2 coated on the VTiMn surface. Part of the Mn element in VTiMn diffused into the Mg matrix, accompanied by the precipitation of nanoscale alpha-Mn particles at high temperature. The improved absorption kinetics can be attributed to the accelerated H atoms diffusion by the microcracks in VTiMn and the phase boundaries provided by the nano-precipitated phase, while the desorption kinetics benefits from the promotion of Mg nucleation and growth by alpha-Mn. Due to the effect of temperature, the rate-determining steps of the hydrogen absorption and desorption process of the composite at 598 K change from H atoms diffusion and Mg nucleation growth to H atoms surface penetration, respectively. These findings will facilitate a more comprehensive understanding of multimetallic catalysts.