Hydrogenation behaviour with regard to storage capacity, kinetics, stability and thermodynamic behaviour of hydrogen storage composite alloys, LaNi5/La2Ni7, LaNi3

Abstract

The present study deals with investigations into the composite hydrogen storage materials LaNi5/La2Ni7, LaNi3, for improved hydrogenation characteristics. To achieve this goal, extensive investigations were carried out to study the effect of the presence of secondary phases on the main phase LaNi5, with reference to hydrogenation behaviour, including incubation time, p-c isotherms, desorption kinetics, hysteresis factor, cyclic stability and thermodynamic behaviour. Alloys were synthesized through melt-casting with varying concentrations of secondary phases like La2Ni7, LaNi3, Ni (minor phases), i.e. 55%, 30%, 5% and 0%, using a special technique of pellet encapsulation. Structural characterization using the XRD technique confirmed the formation of secondary phases together with major phase LaNi5. Microstructural evaluations through SEM revealed that secondary phases suppressed pulverization. Investigations on the hydrogenation behaviour of these alloys showed that the presence of secondary phases in small quantities resulted in a much lower incubation time (lower by 63%), in comparison to singular LaNi5 alloy. The hydrogen storage capacity and desorption kinetics did not decrease for minor concentrations of secondary phases, but a decrease was found for the alloys having higher concentrations of secondary phases. The hysteresis factor has also been found to be less for the multiphasic alloy. The cyclic stability test with regard to storage capacity degradation, showed a higher stability of the alloy having a small concentration of secondary phases. Thermodynamic parameters have been found to be: ΔH (change in enthalpy), -9.05 and -7.03 kcal mol , and ΔS (change in entropy), 31.92 and 23.64 cal mol-1 K-1, for the alloys with 5% and 0% secondary concentrations. © 1999 Elsevier Science S.A. All rights reserved.