Partial substitution of cobalt for nickel in mixed rare earth metal based superlattice hydrogen absorbing alloy – Part 1 structural, hydrogen storage and electrochemical properties

Abstract

The structural, gaseous phase hydrogen storage and electrochemical properties of a series of cobalt-substituted superlattice alloys were investigated as Part 1 of a two-part series. The phase abundances, lattice parameters and unit cell volumes of the AB5, AB3 and A2B7 phases were analyzed by X-ray diffraction patterns. Cobalt (Co) has a more noticeable effect on promoting the AB5 phase than manganese (Mn) and increases both the gaseous phase hydrogen storage and electrochemical capacity of the alloys. It does this by forming more stable hydrides with a lower pressure-concentration-temperature plateau pressure and a more negative heat of formation. The decreasing entropy difference also indicates that Co promotes a more ordered hydride. An alloy with 7 at% Co shows the best high-rate electrochemical performance due to the increased hydrogen diffusion coefficient and exchange current density. Magnetic susceptibility measurements indicate that there are other factors besides the surface nickel (Ni) cluster that dominate the high-rate performance of the Co-substituted superlattice alloys.