Excellent synergistic catalytic mechanism of in-situ formed nanosized Mg2Ni and multiple valence titanium for improved hydrogen desorption properties of magnesium hydride

Abstract

Nowadays, catalytic doping has been regarded as one of the most promising and effective methods to improve the sluggish kinetics of magnesium hydride (MgH2). Herein, we synthesized Ni/TiO2 nanocomposite with the particle sizes about 20 nm by an extremely facile solvothermal method. Then, the Ni/TiO2 nanocomposite was doped into MgH2 to enhance its reversible hydrogen storage properties. A remarkably enhancement of de/rehydrogenation kinetics of MgH2 can be achieved by doped with Ni/TiO2 nanocomposite, compared to that solely doped with Ni or TiO2 nanoparticles. The hydrogen desorption peak temperature of MgH2Ni/TiO2 is 232 °C, which is 135.4 °C lower than that of ball-milled MgH2 (367.4 °C). Moreover, the MgH2Ni/TiO2 can desorb 6.5 wt% H2 within 7 min at 265 °C and absorb ∼5 wt% H2 within 10 min at 100 °C. In particular, the apparent activation energy of MgH2Ni/TiO2 is obviously decreased from 160.5 kJ/mol (ball-milled MgH2) to 43.7 ± 1.5 kJ/mol. Based on the analyses of microstructure evolution, it is proved that metallic Ni particles can react with Mg easily to form fine Mg2Ni particles after dehydrogenation, and the in-situ formed Mg2Ni will transform into Mg2NiH4 in the subsequent rehydrogenation process. The significantly improved hydrogen absorption/desorption properties of MgH2Ni/TiO2 can be ascribed to the synergistic catalytic effect of reversible transformation of Mg2Ni/Mg2NiH4 which act as “hydrogen pump”, and the multiple valence titanium compounds (Ti4+/3+/2+) which promote the electrons transfer of MgH2/Mg.