Hydrogen absorption/desorption mechanism in potassium alanate (KAlH<inf>4</inf>) and enhancement by TiCl<inf>3</inf> doping

Abstract

Thermodynamic and kinetic properties of potassium alanate (KAlH4) are investigated. Its pressure-compositionisotherm measurement exhibits two plateaus for hydrogen absorption/desorption in KAlH4, with gravimetric hydrogen densities of 1.2 ± 0.1 and 2.6 ± 0.2 mass% and reaction enthalpies of 81 and 70 kJmol-1H2, respectively. However, the nonisothermal decomposition of KAlH4occurs through three endothermic events at temperatures of 294, 311, and 347 ° C with the release of hydrogen. Whereas the high temperature event is clearly attributed to K3AlH6decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH4decomposition. FTIR measurements suggest that this intermediate phase is a KyAlHxcompound (y ≥ 1, x ≥ 4) with a high coordination about the aluminum. TiCl3-doped KAlH4also exhibits three decomposition events, but with significant reduction of desorption temperatures (∼ 50 ° C) as well as activation energies that is attributed to particle size reduction and creation of charged vacancies. © 2009 American Chemical Society.