Structure and electrochemical hydrogen storage characteristics of La0.8−x Pr x Mg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0–0.4) electrode alloys

Abstract

For the purpose of improving the electrochemical cycle stability of the La-Mg-Ni based A2B7-type electrode alloys, both reducing Mg content and substituting La with Pr were adopted. The La0.8−xPrxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4) electrode alloys were fabricated by casting and annealing. The investigation on the structures and electrochemical performances of the alloys was performed. The obtained results reveal that the as-cast and annealed alloys comprise two major phases, (La, Mg)2Ni7 phase with the hexagonal Ce2Ni7-type structure and LaNi5 phase with the hexagonal CaCu5-type structure, as well as a little residual LaNi3 phase. It is also found that the addition of Pr element observably affects the electrochemical hydrogen storage characteristics of the alloys, just as the discharge capacity and high rate discharge ability (HRD) first rise then fall with the growing of Pr content, and among all the alloys, the as-cast and annealed (x=0.3) alloys generate the largest discharge capacities of 360.8 and 386.5 mA·h/g, respectively. Additionally, the electrochemical cycle stability of all the alloys markedly grows with the increase of Pr content. The capacity retaining rate (S100) at the 100th charging and discharging cycle is enhanced from 64.98% to 77.55% for the as-cast alloy, and from 76.60% to 95.72% for the as-annealed alloy by rising Pr content from 0 to 0.4. Furthermore, the substitution of Pr for La results in first increase and then decrease in the hydrogen diffusion coefficient (D), the limiting current density (IL) as well as the electrochemical impedance.