Thermochemical transformation and reversible performance of Mg(NH2)2–NaMgH3 system

Abstract

Reversible hydrogen uptake of Mg(NH2)2–MgH2 system has been achieved by the introduction of Na for the first time. The thermochemical and mechanochemical transformations of Mg(NH2)2-xNaMgH3 (x = 0.5, 1 and 2) composites have been investigated through structural characterizations and gas desorption analyses. The hydrogenation property and reaction mechanism of Mg(NH2)2–NaMgH3 have been discussed. Experimental results indicate that the replacement of NaMgH3 for MgH2 can extensively suppress ammonia emission and improve the hydrogen desorption property of Mg(NH2)2–MgH2 system. For Mg(NH2)2–NaMgH3 system, its thermochemical reaction at low temperature is quite similar to the reaction under mechanochemical process. At high temperature (190 °C), the formation of NaMg(NH2)(NH) is detected. The introduction of Na alters the reaction route and enhances the reaction enthalpy of Mg(NH2)2–MgH2 system. The compound of NaMg(NH2)(NH) is synthesized by the reaction of MgNH and NaNH2 and it can reversibly store 1.8 wt% of hydrogen at 10 atm and 190 °C. The reversible cycling between NaMg(NH2)(NH) and Mg(NH2)2 is responsible for the partial reversible hydrogenation of Mg(NH2)2–NaMgH3 system.