for on board h 2 storage are equilibrium reactions that alternately store and release h 2 according to the phase temperature and pressure of the system one class of such chemical reactions is those involving metal hydrides metal hydrides undergo reversible chemical reactions which under conditions of temperature and pressure that are sometimes suitable for on board vehicles either absorb or release hydrogen the parent metallics of metal hydrides typically take any of the following forms a ab 5 ab 2 ab a 2 b where a and b are typically a mix of transition metals or a mix of transition with either rare earth or alkaline earth metals table i shows examples of metal hydrides and their hydrogen storage properties the formation of a metal hydride from the reaction of hydrogen with a parent metallic s proceeds by a set of well characterized processes hydrogen molecules first dissociate on the surface before the dissociated hydrogen dissolves in the host metal residing in the interstitial sites of the host metal crystal structure but the molecular hydrogen must be able to contact the surface of pure metal hydride for dissociation and dissolution to take place in other words the surface of a metal hydride precursor material must be free of films that inhibit hydrogen absorption at higher hydrogen pressure.