An overview of the different types of metal hydrides and complex hydrides being investigated for on-board (reversible) and off-board (non-reversible) hydrogen storage will be presented along with a few new approaches to improving the hydrogenation- dehydrogenation properties. In the interest of high gravimetric capacities, much of the current metal hydride reasearch is focused on hydrides composed primarily of light elements (Z ≤ 13). For reversible hydrides, much attention has recently been devoted to the complex hydrides, where the hydrogen atoms are covalently bonded to a central atom in an anion complex (e.g. [AlH6]3-, [AlH4]-, [BH4]-, [NH2]-) and stabilized by a cation, typically an alkali (e.g. Li) or alkaline earth metal (e.g. Mg). These compounds have appreciable gravimetric hydrogen storage capacities, but are plaqued by slow kinetics and high decomposition temperatures. 
 
The development of an off-board (non-reversible) hydrogen storage system is also facing a unique set of challenges, but this approach may have a couple of important advantages. For example, the kinetically stabilized hydrides (e.g. AlH3, LiAlH4) are well suited to low temperature automotive fuel cells since they require minimal heat input during decomposition and offer rapid, low temperature hydrogen evolution rates in addition to their high capacities. These metastable hydrides have traditionally been overlooked since they cannot be re-hydrogenated under moderate hydrogen pressures and low cost chemical regeneration methods have remained elusive. Recently, a number of new solution phase and electrochemical processes have been proposed to reform these metastable hydrides from the spent fuel and H2 gas at moderate pressures and temperatures.