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/H2 atmosphere, biphasic nanoparticles are grown, in which the bulk-immiscible MgH2 and TiH2 phases are mixed at the nanoscale. TiH2 conveys catalytic activity for H2 dissociation/recombination and accelerated hydrogen diffusion, while MgH2 provides reversible hydrogen storage. At the remarkably low temperature of 150 °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 °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 Δγ=(0.38±0.04)Jm−2 between hydride and metal phases at T = 100 °C. These unique composite nanoparticles significantly extend the temperature/pressure window of hydrogen storage applications using Mg-based materials compatible with up-scaling.