Effect of Cr substitution by Ni on the cycling stability of Mg2Ni alloy using EXAFS

Abstract

This paper describes the hydrogen storage properties of Mg2Ni0.9Cr0.1 alloy and aims to elucidate the effect of doping Cr on the hydrogen sorption/desorption kinetics upon cycling. Mg2Ni0.9Cr0.1 alloy shows stable absorption capacity, and its absorption/desorption rates further improve after cycling. The calculated activation energy for dehydrogenation was 53 kJ/mol at the 3rd cycle, and decreased to 36 kJ/mol at the 20th cycle. XRD combined with SEM exhibits that Cr dopant substitutes for Mg or Ni after ball milling and the lattice structure remains stable over 20 cycles. EXAFS was used to investigate the local coordination of Ni and Cr atoms in the ball-milled and cycled samples. For the ball-milled sample, the strong Cr–Ni bonds weaken the Cr–Mg bonds, thereby destabilizing all Cr-doped phases. After 20 cycles, the stable Ni1–Mg1 bonds may be dominant and control the structural stability of Mg2Ni phases.