Evaluation techniques of cycling effect on thermodynamic and crystal structure properties of Mg2Ni alloy

Abstract

Hydrogen storage using reversible lightweight metal hydrides can open the way towards hydrogen utilization for energy applications like car fuel. Magnesium-nickel alloys are the most promising lightweight materials which offer a favorable volumetric hydrogen storage density. For a vehicular application, a prime consideration is the stability of the hydrogen absorbing/desorbing capacity of the hydride when it is subjected to a large number of absorption/desorption cycles. In order to estimate if magnesium-nickel alloys would be attractive candidates for practical hydrogen storage systems, a new and rapid experimental device was designed for fast continuous evaluation of the cyclic charge and discharge stability of the intermetallic compound Mg2Ni over a relatively large number of cycles. Repetitive hydriding and dehydriding of a metal alloy is simply performed by cycling the sample from the high-pressure circuit used for absorption to the low pressure circuit having enough volume to receive the hydrogen discharge. Simultaneously the sample holder containing the hydride may be alternated from the low temperature furnace for absorption to the high temperature furnace for desorption. The cycling stability of commercial crystalline Mg2Ni alloy was tested at 300°C over up to 2700 cycles. Periodically during the cycling experiments, we have also performed measurements of the dynamic hydrogen absorption/desorption; these were done at the reaction temperatures of 250°C and 300°C, respectively. Moreover, pressure composition isotherms at 300°C measurements were also carried out. The cycling effect was also evaluated via additional experiments involving SEM, Brunauer-Emmett-Teller (BET) specific surface area, specific heat and X-ray crystal structure analysis.