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.

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery. Here we present the mechanism of Mg-ion conductivity of monoammine magnesium borohydride, Mg(BH₄)₂·NH₃. Density functional theory calculations (DFT) reveal that the neutral molecule (NH₃) in Mg(BH₄)₂·NH₃ is exchanged between the lattice and interstitial Mg²⁺ facilitated by a highly flexible structure, mainly owing to a network of di-hydrogen bonds, N–H^δ+⋯^−δH–B and the versatile coordination of the BH₄ ⁻ ligand. DFT shows that di-hydrogen bonds in inorganic matter and hydrogen bonds in bio-materials have similar bond strengths and bond lengths. As a result of the high structural flexibiliy, the Mg-ion conductivity is dramatically improved at moderate temperature, e.g. σ(Mg²⁺) = 3.3 × 10⁻⁴ S cm⁻¹ at T = 80 °C for Mg(BH₄)₂·NH₃, which is approximately 8 orders of magnitude higher than that of Mg(BH₄)₂. Our results may inspire a new approach for the design and discovery of unprecedented multivalent ion conductors.

A new hemiammine borohydride, LiBH₄·1/2NH₃, has been presented, which exhibits fast Li-ion conduction promoted by ammonia.

Efficient storage of hydrogen is one of the biggest challenges towards a potential hydrogen economy. Hydrogen storage in liquid carriers is an attractive alternative to compression or liquefaction at low temperatures. Liquid carriers can be stored cost-effectively and transportation and distribution can be integrated into existing infrastructures. The development of efficient liquid carriers is part of the work of the International Energy Agency Task 40: Hydrogen-Based Energy Storage. Here, we report the state-of-the-art for ammonia and closed CO₂-cycle methanol-based storage options as well for liquid organic hydrogen carriers.