Kinetics of Hydrogen Desorption in NaAlH₄ and Ti-Containing NaAlH₄

Abstract

Understanding the fundamental reaction path and identifying the rate-limiting steps in the decomposition of materials evolving hydrogen are two challenging tasks when studying complex metal hydrides as viable onboard hydrogen storage materials. In this work, we use computational techniques to study the free-energy barriers associated with the reactions involved in the evolution of hydrogen via the first step of sodium alanate decomposition (NaAlH4 → 1/3Na3AlH6 + 2/3Al + H2). Results from our calculations suggest a four-step reaction that includes the transition from AlH4− to AlH63− anions, Al clustering, and H2 evolution. The calculated free-energy barrier and enthalpy of activation associated with one molecule of H2 release are on the order of 80 and 82 kJ/mol H2, respectively. The rate-determining step for this mechanism is found to be the hydrogen evolution from associated AlH3 species. The role of titanium in the improved kinetics of Ti-containing sodium alanates is elucidated from our coupled density functional theory/molecular dynamics calculations. Ti stays on the hydride surface and serves as both the catalytic species in splitting hydrogen from AlH4/AlH3 groups as well as the initiator for Al nucleation sites in Ti-doped NaAlH4.