aDalian Institute of Chemical Physics, Dalian, PR China 116023
bDepartment of Physics and Department of Chemistry, National University of Singapore, Singapore 117542
cSchool of Materials Science and Engineering, South China University of Technology, Guangzhou, PR China 510641
Available online 27 November 2008.
The ever-increasing demand for energy coupled with dwindling fossil fuel resources make the establishment of a clean and sustainable energy system a compelling need. Hydrogen-based energy systems offer potential solutions. Although, in the long-term, the ultimate technological challenge is large-scale hydrogen production from renewable sources, the pressing issue is how to store hydrogen efficiently on board hydrogen fuel-cell vehicles and .
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 , and from activated carbon to carbon nanotubes and metal organic frameworks and . The use of advanced synthetic routes also yields a range of physical states from bulk crystalline structures to amorphous states to nanostructures9. 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 and . 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 and . 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 and (see alsoTable 2). 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 NaBH4and NH3BH3, is beyond the scope of this review.
Summary of the complex hydrides and chemical hydrides
Some transition metals and their alloys react with gaseous hydrogen or H atoms from electrolytes to form metal hydrides; such reversible hydriding reactions render these metals and their alloys potential hydrogen storage materials. As an example, the volumetric hydrogen density of LaNi5H6 is 115 kg/m3 which is much higher than that of compressed hydrogen or liquid hydrogen.
Typical metal hydrides and their properties are summarized in Table 1. Except for MgH2 and Mg2NiH4, the gravimetric hydrogen density of metal hydrides is usually less than 3.0 wt.%, while the equilibrium hydrogen pressures (Peq) for hydrogen desorption at temperature T vary with their compositions. Specific matching of metals with different affinities to hydrogen allows the properties of ternary hydrides to be altered, and in fact provides an important alloying guideline for metal hydrides. For instance, elemental substitutions of LaNi5 and ZrV2 alloys are used to improve the electrochemical hydrogen storage properties of these materials; addition of light elements such as Mg or Ca into LaNi3 alloys leads to alloys with higher hydrogen storage capacities, e.g. RE–Mg–Ni alloy (RE = Rare Earth) with a gravimetric density of about 1.8%16 and LaMg3 alloy with a gravimetric density of about 3%17; low-cost Fe addition into Ti–10Cr–18Mn–32V solid solution alloy increases the equilibrium pressure and capacity18.
Structure and hydrogen storage properties of typical metal hydrides
Although they have remarkably high volumetric density and suitable thermodynamic and kinetic properties, transition metals and their alloys are not recommended for onboard hydrogen storage applications mainly due to their relatively low gravimetric hydrogen density. In the following part, we will focus on the recent developments in Mg-based materials with an emphasis on nanostructure, nanocatalysis and nanocomposite formation.
Experimental results show that grain refining, especially to the nanoscale19, significantly improves the kinetics of hydriding and dehydriding of metals and alloys. Ball milling is the most widely used method to refine the grains of hydrides. Nanocrystalline Mg9 and Mg2Ni20 obtained by ball milling exhibit much faster hydrogen sorption rates than their bulk counterparts at relatively low temperatures due to the enhanced surface effect and shortened diffusion path. However, it has been shown in some cases that reducing grain size to the nanometer scale also decreases reversible storage capacity due to the reduction of intragrain volume21. Recent work shows that reducing the size of Mg-based hydrides to the nanometer scale can also alter their stability, which can be characterized by the desorption energy (the energy needed for dissociation of