Catalytic mechanisms of nickel nanoparticles for the improved dehydriding kinetics of magnesium hydride

Abstract

MgH₂, albeit with slow desorption kinetics, has been extensively studied as one of the most ideal solid hydrogen storage materials. Adding such catalyst as Ni can improve the desorption kinetics of MgH₂, whereas the catalytic role has been attributed to different substances such as Ni, Mg₂Ni, Mg₂NiH_0.3, and Mg₂NiH₄. In the present study, Ni nanoparticles (Ni-NPs) supported on mesoporous carbon (Ni@C) have been synthesized to improve the hydrogen desorption kinetics of MgH₂. The utilization of Ni@C largely decreases the dehydrogenation activation energy from 176.9 to 79.3 kJ mol⁻¹ and the peak temperature of dehydrogenation from 375.5 to 235 °C. The mechanism of Ni catalyst is well examined by advanced aberration-corrected environmental transmission electron microscopy and/or x-ray diffraction. During the first dehydrogenation, detailed microstructural studies reveal that the decomposition of MgH₂ is initially triggered by the Ni-NPs, which is the rate-limiting step. Subsequently, the generated Mg reacts rapidly with Ni-NPs to form Mg₂Ni, which further promotes the dehydrogenation of residual MgH₂. In the following dehydrogenation cycle, Mg₂NiH₄ can rapidly decompose into Mg₂Ni, which continuously promotes the decomposition of MgH₂. Our study not only elucidates the mechanism of Ni catalyst but also helps design and assemble catalysts with improved dehydriding kinetics of MgH₂.