On the Structural Evolution of a Magnesium-Carbon Composite for Hydrogen Storage on Extended Cycling
Magnesium , Hydrogen Storage , Structural Evolution , Magnesium Hydride
Magnesium is an attractive material for solid-state hydrogen storage (SSHS) applications owing to its reversible hydride formation, high theoretical gravimetric hydrogen capacity, natural abundance, and low cost. A significant body of research has been done on the use of magnesium for this application over several decades. To date, no systematic structural evolution study has been reported which combines high cycle count, periodic sampling, and microstructural analysis. In this study, a candidate material was brought to more than 1000 hydrogen sorption cycles with a predetermined set of sampling points intended to capture the major morphological and performative states. Several samples were obtained during the early cycles to capture the activation process, with sampling continuing up to and including the 1000th cycle. The material selected for this experiment was 95 wt. % magnesium and 5 wt. % carbon prepared by high energy ball milling (HEBM). Approximately 2 g of the material was placed in an in-house hydriding rig at Canadian Nuclear Laboratories (CNL) which automatically cycled the material between hydrogen absorption with 15 bar(a) onset pressure and hydrogen desorption with 0.02 bar(a) onset pressure. Samples removed from the reactor were analyzed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and focused ion beam (FIB) assisted SEM and energy-dispersive X-ray spectroscopy (EDX). Data from these analyses was combined with performance data from cycling with the aim of finding correlations between material performance and structural evolution. The results show a clear evolution of the microstructure in two stages. The first stage results in the rapid transformation of the as-milled material to form the activated structure, a branching ligamented structure formed by ‘c’-directional growth of the Mg phase on successive hydride decompositions. Herein, the activation process is delineated into two distinct parts: functional and morphological activation. The second stage of structural evolution involves the slow agglomeration and densification of the activated structure via sintering, eventually leading to kinetic limitations which reduce the useful lifetime of the material. The material remained viable after 1000 cycles, retaining nearly 88% of its peak hydrogen capacity, which is suitable for many end-use applications.