Three dimensional-transition element selectivity for electrochemical hydrogen charging in MgAlNi-based lightweight high-entropy alloys as electrodes for M-MH batteries

• Ti, Cr, Fe, Co, Zn and Cu formed BCC-type structures in MgAlNi-based HEAs. • Co, Zn and Cu improve hydrogen discharge capacity in MgAlNi-based HEAs. • Co improves the hydrogen diffusion charge/discharge kinetics of MgAlNi-based HEAs. • Cr and Zn have a negative effect on the lifetime of MgAlNi based HEAs. • Fe and Ti influence the hydrogen diffusion mechanism of MgAlNi-based HEAs. This study reports the effect of 3d transition elements on the electrochemical hydrogen storage of MgNiAl-based high-entropy alloys (HEAs). The HEAs were designed using empirical equations that relate thermodynamic parameters such as enthalpy and valence electron concentration. The HEAs of the MgAlNiTiCr, MgAlNiTiCo, MgAlNiFeTi, MgAlNiFeCu and MgAlNiFeZn alloys systems were obtained by high-energy ball-milling. The electrochemical hydrogen storage properties investigated were the activation capacity, discharge capacity, charge-discharge kinetics, corrosion-passivation, and hydrogen diffusion mechanism. The design equations for the HEAs predicted the formation of a solid solution with a body-centered cubic structure as the main phase, which was confirmed after structural and microstructural characterization. Alloys with Ti presence favors the TiH 2 formation as a secondary phase during the milling process. The HEAs revealed good activation properties for electrochemical hydrogen storage. The electrochemical analysis results of the HEAs porous electrodes showed that Cr inhibits the electrode discharge capacity, Co improves the charge/discharge kinetics, Ti and Fe influence the diffusion mechanism, Cu significantly increases the hydrogen discharge capacity and Zn provides a good discharge capacity and facilitates the corrosion-oxidation of the electrode surface, similar to Cr.