Effects of additive Pd on the structures and electrochemical hydrogen storage properties of Mg₆₇Co₃₃-based composites or alloys with BCC phase

Highlights: • Additive Pd in Mg₆₇Co₃₃ benefits to form a ternary BCC alloy. • Introducing 5.0 at.% Pd in Mg₆₇Co₃₃ lifts the initial discharge capacity from 10 mAh/g to maximum 530 mAh/g. • Exchange current density was increased due to the homogeneously dispersed Pd. • Additive Pd slightly enhances the hydrogen diffusion coefficient of Mg-Co-Pd composites or alloys. - Abstract: Mg₆₇Co₃₃ and Mg₆₇Co₃₃-Pd composites/alloys prepared by ball milling for 120 h possess nano-crystalline with body-centered cubic (BCC) structure, which was verified by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses. The introduced 5.0 at.% Pd significantly lifts the initial discharge capacity from 10 mAh g^-1 of Mg₆₇Co₃₃ to maximum 530 mAh g^-1. Pd also drives the Mg₆₇Co₃₃-Pd composite forming a full BCC alloy during ball milling. The distribution of Pd gradually becomes homogeneous with the augmentation of the ball milling time according to the analyses by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). Exchange current density increased with the milling time and can be ascribed to the homogeneously dispersion of Pd over the surface. The introduced Pd also enhances the hydrogen diffusion coefficient of the Mg₆₇Co₃₃-Pd composites/alloys