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

Abstract

Thermodynamic and kinetic properties of potassium alanate (KAlH 4 ) are investigated. Its pressure-compositionisotherm 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 g 1, x g 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.

Novel hydrogen storage materials: A review of lightweight complex hydrides

“…These parameters were found to be consistent with those reported in the experimental data [120]. They also predicted that the KAlH 4 ground state structure consists of slightly distorted [AlH 4 − ] tetrahedra ( Fig.…”

“…The enthalpies of formation of KAlH 4 and K 3 AlH 6 were found to be −70 and −78.5 kJ/mol, while the enthalpies of decomposition, reactions equations (22) and (23), were determined as 55 and 70 kJ/mol H 2 , respectively [118]. Recently, a significant reduction in desorption temperature (∼50 • C) of the reaction KAlH 4 → K 3 AlH 6 was reported due to TiCl 3 doping [120]. However, TiCl 3 does not affect the decomposition temperature of K 3 AlH 6 .…”

Synthesis of calcium alanate and its dehydriding performance enhanced by FeF3 doping

“…LiAlH 4 has a theoretical capacity of 10.6 wt.% H 2 , the direct hydrogenation of LiAlH 4 is difficult to be achieved because of its unstable thermodynamics. The decomposition of KAlH 4 occurs through three endothermic events at temperatures of 294, 311, and 347 • C with the release of 3.8 wt.% hydrogen, and its catalyzed sample exhibits dehydrogenation temperatures dropped by nearly 50 • C, however, which are still rather high when compared to sodium and lithium alanates [8]. Ca(AlH 4 ) 2 with a total hydrogen capacity of 7.9 wt.% has been far less studied.…”

“…The structural properties of Ca(AlH 4 ) 2 and CaAlH 5 have been determined by first-principle density function theory (DFT) calculation in the past few years. Crystal structure of Ca(AlH 4 ) 2 is confirmed to be CaB 2 F 8 cal chains [18][19][20][21]. From the foregoing, attempts to improve the properties of Ca(AlH 4 ) 2 and CaAlH 5 , such as doping or element substitute, are also important.…”

Complex Metal Hydrides for Hydrogen, Thermal and Electrochemical Energy Storage

“…The formation of an intermediate has been observed experimentally and a crystal structure and composition of K 3 AlH 6 has been suggested by density functional theory (DFT) calculations [37,[81][82][83]. In contrast, experimental observations suggest a K y AlH x compound with 1 ≤ y ≤ 3 and 4 ≤ x ≤ 6 [57].…”