ALLOYS FOR HYDROGEN STORAGE IN NICKEL-HYDROGEN AND NICKEL METAL HYDRIDE BATTERIES

Since 1990, there has been an ongoing collaboration among the authors in the three laboratories (i) to prepare alloys of the AB(5) and AB(2) types, using arc-melting/annealing and mechanical alloying/annealing techniques; (ii) to examine their physicochemical characteristics (morphology, composition; (iii) to determine the hydrogen absorption/desorption behavior (pressure-composition isotherm as a function of temperature), and (iv) to evaluate their performance characteristics as hydride electrodes (charge/discharge, capacity retention, cycle life, high-rate capability). This review article presents the work carried out on representative AB(5) and AB(2) type modified alloys (by partial substitution of with small additives of other elements). The purpose of the modification was to optimize the thermodynamics and kinetics of the hydriding/dehydriding reactions and to enhance the stabilities of the alloys for the desired battery applications. The results of our collaboration, to date, demonstrate that: (i) alloys prepared by arc-melting/annealing and mechanical alloying/annealing techniques exhibit similar morphology, composition and hydriding/dehydriding characteristics; (ii) alloys with the appropriate small amounts of substituent or additive elements - retain the single phase structure, improve the hydriding/dehydriding reactions for the battery applications, and enhance the stability in the battery environment - and (iii) the AB(2) type alloys exhibit higher energy densities than the AB(5) type alloy but the state-of-the-art, commercialized batteries are predominantly manufactured using AB(5) type alloys.