2Mg–Fe and 2Mg–Fe + 5%C mixtures processed by hot extrusion: Influence of carbon on hydrogen sorption properties

Abstract

Mg is a promising candidate material for hydrogen storage but its application is restricted by high sorption temperatures and slow sorption kinetics. Major efforts have focused on overcoming these two obstacles. Alloying, nanostructuring and/or catalysts, particularly at low temperatures, have considerably improved the hydrogenation kinetics of Mg powders. Further investigation is still desirable to reach the same properties for bulk materials because hydrogen desorption in bulk Mg-based systems still remains at a critical high temperature, and this, represents a major obstacle to their practical application. The aim of this work was to evaluate the reduction in desorption temperature and the increase in hydrogenation kinetics of bulk Mg-based composites in response to the following variables: particle/grain size, density of defects, the use of iron as catalyst, volume modifications, and the presence of carbon as a pore-confining element. We compared the behavior of 2Mg–Fe powder mixtures produced by high-energy ball milling processed as follows to produce bulk samples: (i) hot extrusion of 2Mg–Fe with extrusion ratio of 5/1 (S-1), (ii) hot compaction of 2Mg–Fe (S-2), and (iii) hot compaction of ball-milled 2Mg–Fe + 5%C, all processed at 300 °C (S-3). The samples were activated and subsequently hydrogenated at 15 bar, sample S-1 for 24 h and sample S-2 and S-3 for 5 h. After the hydrogenation treatments, the complex hydride Mg 2FeH 6 was formed in all the conditions. Desorption temperatures varied according to the processing conditions and the results indicated that porosity was an important factor, as it was the addition of carbon, which accelerated the desorption kinetics.