Tremendous efforts have been devoted to the research and development of materials that can hold sufficient hydrogen in terms of gravimetric and volumetric densities, and, at the same time, possess suitable thermodynamic and kinetic properties. Over approximately a decade of exploration, the scope of candidate materials has expanded greatly, from traditional metal hydrides to complex hydrides and chemical hydrides, and from activated carbon to carbon nanotubes and metal organic frameworks. The use of advanced synthetic routes also yields a range of physical states from bulk crystalline structures to amorphous states to nanostructures. Simulations are having an increasing impact not only on the description of physical properties of known materials, but also on the prediction of novel structures and reaction paths.

In this review, we will focus on the recent progress in metal hydrides and complex hydrides. Chemical hydrides, represented by ammonia borane, have attracted increasing interest, and a few reviews have addressed the thermal decomposition, catalyst development, off-board regeneration, etc., of these materials. More recently, a novel approach to optimizing ammonia borane for hydrogen storage has been developed. By replacing one H in ammonia borane by an alkali or alkaline earth element, a new family of compounds, named amidoborane, has been synthesized; these compounds have high hydrogen contents and relatively low dehydrogenation temperatures. There is also exciting progress in sorbent materials research (such as Metal–Organic Frameworks (MOFs)), which, we think, merits a separate review. Hydrolysis of chemical hydrides, such as NaBH₄ and NH₃BH₃, is beyond the scope of this review.