Microstructural evolution during hydrogen sorption cycling of Mg–FeTi nanolayered composites

This paper describes the microstructural evolution of Mg–FeTi mutlilayered hydrogen storage materials during extended cycling. A 28 nm Mg–5 nm FeTi multilayer has comparable performance to a cosputtered material with an equivalent composition (Mg–10%Fe–10%Ti), which is included as a baseline case. At 200 °C, the FeTi layers act as a barrier, preventing agglomeration of Mg particles. At 300 °C, the initial structure of the multilayer is preserved up to 35 cycles, followed by fracturing of the Mg layers in the in-plane direction and progressive delamination of the FeTi layers as observed by electron microscopy. Concurrently, an increase in the Mg grain size was observed from 32 to 76 nm between cycles 35 and 300. As a result, the absorption kinetics deteriorate with cycling, although 90% of the total capacity is still absorbed within 2 min after as many as 300 cycles. The desorption kinetics, on the other hand, remain rapid and stable, and complete desorption of 4.6 wt.% H is achieved in 1.5 min at ambient desorption pressure. In addition to showing good hydrogen storage performance, multilayers are an excellent model system for studying the relation between microstructure and hydrogen absorption/desorption kinetics.