Hydrogen Absorption/Desorption Mechanism in Potassium Alanate (KAlH4) and Enhancement by TiCl3Doping

Thermodynamic and kinetic properties of potassium alanate (KAlH4) are investigated. Its pressure-composition-isotherm 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 kJ.mol(-1) H-2, respectively. However, the nonisothermal decomposition of KAlH4 occurs through three endothermic events at temperatures of 294, 311, and 347 degrees C with the release of hydrogen. Whereas the high temperature event is clearly attributed to K3AlH6 decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH4 decomposition. FTIR measurements suggest that this intermediate phase is a KyAlHx compound (y >= 1, x >= 4) with a high coordination about the aluminum. TiCl3-doped KAlH4 also exhibits three decomposition events, but with significant reduction of desorption temperatures (similar to 50 degrees C) as well as activation energies that is attributed to particle size reduction and creation of charged vacancies.