Interfaces within biphasic nanoparticles give a boost to magnesium-based hydrogen storage

The use of Mg as hydrogen storage material is hampered by slow sorption kinetics and high thermodynamic stability. This work reports on biphasic Mg-Ti-H nanoparticles that outperform known Mg-based materials in both respects. By exploiting gas-phase condensation of Mg and Ti vapors under He/H-2 atmosphere, biphasic nanoparticles are grown, in which the bulk-immiscible MgH2 and TiH2 phases are mixed at the nanoscale. TiH2 conveys catalytic activity for H-2 dissociation/recombination and accelerated hydrogen diffusion, while MgH2 provides reversible hydrogen storage. At the remarkably low temperature of 150 degrees C, hydrogen absorption and desorption are completed in less than 100 s and 1000 s, respectively. Moreover, the equilibrium pressure for hydrogen sorption exhibits a composition-dependent upward shift compared to bulk Mg, resulting in a pressure increase by a factor of about 4.5 in the Ti-richest samples at 100 degrees C. The enthalpy and entropy of the metal-hydride transformation are both lower in magnitude with respect to the bulk values, suggesting opposite contributions to the free energy change. The results are analyzed by an interface-induced hydride destabilization model, determining an interfacial free energy difference Delta gamma = (0.38 +/- 0.04) Jm(-2) between hydride and metal phases at T = 100 degrees C. These unique composite nanoparticles significantly extend the temperature/pressure window of hydrogen storage applications using Mg-based materials compatible with up-scaling.