Hydrogen storage in MgAlTiFeNi high entropy alloy

Highlights: • Reactive milled MgAlTiFeNi HEA showed a hydrogen storage capacity of 0.94 wt%. • Hydrogen sorption kinetics of reactive milled HEA is high. • Electrochemical discharge capacity of reactive milled HEA was 257 mAh/g. • Reactive milling preactivation enhances electrochemical hydrogen sorption properties. • Hydroxide films impair electrochemical reactions on the HEA surface. -- Abstract: In this study, the MgAlTiFeNi high entropy alloy was processed by high-energy ball milling under both argon and hydrogen atmospheres. It is shown that this alloy forms a body-centered cubic (BCC) structure when milled under an argon atmosphere (mechanical alloying-MA) and a combination of BCC, FCC, and Mg₂FeH₆ when milled under hydrogen pressure (reactive milling-RM). The hydrogen storage behavior of the RM-MgAlTiFeNi samples was evaluated by a combination of thermal analyses and manometric measurements in a Sieverts apparatus. The RM-MgAlTiFeNi alloy presented an initial functional hydrogen storage capacity of 0.87 wt%, which increased to 0.94 wt% after the second absorption. Also, it exhibited a high hydrogen absorption and desorption kinetics at temperatures 100 °C lower than the one for the desorption temperature of the commercial MgH₂. Electrochemical discharge of RM-MgAlTiFeNi samples showed precisely the same hydrogen content as that obtained in the gas desorption. Electrochemical charging/discharging experiments were also performed in the MA-MgAlTiFeNi samples, which presented lower electrochemical storage capacity, a behavior probably resulting from the instability of the alloy in the alkaline solution with the formation of a hydroxide layer on its surface that hinders the electrochemical reactions.