Achieving room temperature hydrogen storage reversibility in Nb-rich alloys of the Nb-Cr-Mn system

Abstract

Recent developments in the design of body-centered cubic (BCC) multicomponent alloys via computational tools have demonstrated the possibility of obtaining alloys with excellent hydrogen storage behavior. In this work, we employ the CALPHAD (Calculation of Phase Diagrams) method to design Nb-rich alloys of the Nb-Cr-Mn system that present hydrogen storage reversibility at room temperature under moderate pressure conditions. We employ the valence electron concentration (VEC) factor as a compositional guide to select compositions with suitable thermodynamic properties. Using electric arc melting, we synthesize two alloys, namely Nb85Cr10Mn5 and Nb70Cr20Mn10, both forming predominant BCC solid solutions with VEC ~ 5.2 and minor amounts of a eutectic microconstituent composed of BCC and Laves C14 phases. Both alloys are easily hydrogenated at room temperature without the need for an activation treatment. The Nb85Cr10Mn5 alloy reaches a storage capacity of 2.1wt.% of H (298K; Peq~ 20bar) whereas the Nb70Cr20Mn10 alloy reaches a capacity of 1.4wt.% (298K; Peq~ 21bar). Benefits in the storage kinetic performance are correlated with the BCC + C14 microstructure. Pressure-Composition-Temperature (PCT) diagrams show moderate values of equilibrium pressure for hydrogen storage reversibility at room temperature. Room temperature absorption/desorption cycling measurements demonstrated a reversible capacity of 1.2wt.% of H (Peq~ 29bar) for the Nb85Cr10Mn5 alloy and 0.8wt.% of H (Peq~ 31bar) for the Nb70Cr20Mn10 alloy after twenty cycles.