Intense literature and research efforts have focussed on the exploration of complex hydrides for energy storage applications over the past decades. A focus was dedicated to the determination of their thermodynamic and hydrogen storage properties, due to their high gravimetric and volumetric hydrogen storage capacities, but their application has been limited because of harsh working conditions for reversible hydrogen release and uptake. The present review aims at appraising the recent advances on different complex hydride systems, coming from the proficient collaborative activities in the past years from the research groups led by the experts of the Task 40 ‘Energy Storage and Conversion Based on Hydrogen’ of the Hydrogen Technology Collaboration Programme of the International Energy Agency. An overview of materials design, synthesis, tailoring and modelling approaches, hydrogen release and uptake mechanisms and thermodynamic aspects are reviewed to define new trends and suggest new possible applications for these highly tuneable materials.

The system Mg(NH₂)₂ + 2LiH is considered as an interesting solid-state hydrogen storage material owing to its low thermodynamic stability of ca. 40 kJ/mol H₂ and high gravimetric hydrogen capacity of 5.6 wt.%. However, high kinetic barriers lead to slow absorption/desorption rates even at relatively high temperatures (>180 °C). In this work, we investigate the effects of the addition of K-modified Li_xTi_yO_z on the absorption/desorption behaviour of the Mg(NH₂)₂ + 2LiH system. In comparison with the pristine Mg(NH₂)₂ + 2LiH, the system containing a tiny amount of nanostructured K-modified Li_xTi_yO_z shows enhanced absorption/desorption behaviour. The doped material presents a sensibly reduced (∼30 °C) desorption onset temperature, notably shorter hydrogen absorption/desorption times and reversible hydrogen capacity of about 3 wt.% H₂ upon cycling. Studies on the absorption/desorption processes and micro/nanostructural characterizations of the Mg(NH₂)₂ + 2LiH + K-modified Li_xTi_yO_z system hint to the fact that the presence of in situ formed nanostructure K₂TiO₃ is the main responsible for the observed improved kinetic behaviour.

Rechargeable solid-state magnesium batteries are considered for high energy density storage and usage in mobile applications as well as to store energy from intermittent energy sources, triggering intense research for suitable electrode and electrolyte materials. Recently, magnesium borohydride, Mg(BH₄)₂, was found to be an effective precursor for solid-state Mg-ion conductors. During the mechanochemical synthesis of these Mg-ion conductors, amorphous Mg(BH₄)₂ is typically formed and it was postulated that this amorphous phase promotes the conductivity. Here, electrochemical impedance spectroscopy of as-received γ-Mg(BH₄)₂ and ball milled, amorphous Mg(BH₄)₂ confirmed that the conductivity of the latter is ~2 orders of magnitude higher than in as-received γ-Mg(BH₄)₂ at 353 K. Pair distribution function (PDF) analysis of the local structure shows striking similarities up to a length scale of 5.1 Å, suggesting similar conduction pathways in both the crystalline and amorphous sample. Up to 12.27 Å the PDF indicates that a 3D net of interpenetrating channels might still be present in the amorphous phase although less ordered compared to the as-received γ-phase. However, quasi elastic neutron scattering experiments (QENS) were used to study the rotational mobility of the [BH₄] units, revealing a much larger fraction of activated [BH₄] rotations in amorphous Mg(BH₄)₂. These findings suggest that the conduction process in amorphous Mg(BH₄)₂ is supported by stronger rotational mobility, which is proposed to be the so-called “paddle-wheel” mechanism.