TiH₂ as a Dynamic Additive for Improving the De/Rehydrogenation Properties of MgH₂: A Combined Experimental and Theoretical Mechanistic Investigation

We report a mechanism for how low concentrations of TiH2 can function as a dynamic additive for improving both the kinetics and the thermodynamics of the Mg/MgH2–TiH2 system. We use a combination of experimental and theoretical methods to study de/rehydrogenation in Mg/MgH2–TiH2 systems. Our experiments show that ball milling MgH2 with 5 wt % TiH2 improves the thermodynamics and kinetics of de/rehydrogenation in Mg/MgH2, and these improvements are robust with respect to cycling. The enthalpy change for the Mg/MgH2–TiH2 system is about 7 kJ/mol H2 lower than that for ball-milled MgH2. Structural (X-ray diffraction, in situ X-ray diffraction) and microstructural (electron microscopy) characterization has been employed to understand the role and state of the catalyst. We have developed a theoretical model that broadly captures the experimental observations and performed first-principles density functional theory and phonon density of states calculations to identify the energetically preferred location of TiH2 in the Mg/MgH2–TiH2 systems. We found that TiH2 located beneath the surface of Mg is energetically more favorable than being located on top of Mg. In contrast, TiH2 on the surface of MgH2 is favored over subsurface configurations. We hypothesize that TiH2 particles migrate from the subsurface to the surface during hydrogenation of Mg and in the opposite direction during dehydrogenation of MgH2. The hypothesis of migration of TiH2 particles during de/rehydrogenation of Mg/MgH2–TiH2 is supported by our microstructural (electron microscopy) and XPS studies.