Hydrogen Absorption/Desorption Mechanism in Potassium Alanate (KAlH4) and Enhancement by TiCl3 Doping

Thermodynamic and kinetic properties of potassium alanate (KAlH 4 ) are investigated. Its pressure−composition-isotherm measurement exhibits two plateaus for hydrogen absorption/desorption in KAlH 4, 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 KAlH 4 occurs 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 K 3 AlH 6 decomposition, the low temperature events occur by two reactions, denoting the existence of an intermediate phase during KAlH 4 decomposition. FTIR measurements suggest that this intermediate phase is a K y AlH x compound ( y ≥ 1, x ≥ 4) with a high coordination about the aluminum. TiCl 3 -doped KAlH 4 also 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.