Annealing effects on structural and electrochemical properties of (LaPrNdZr)0.83Mg0.17(NiCoAlMn)3.3 alloy

Rare-earth magnesium nickel based alloys (La0.205Pr0.392Nd0.392Zr0.012)0.83Mg0.17Ni2.97Co0.1Al0.2Mn0.03 were studied with various annealing conditions. The phase structures of the alloys before and after electrochemical cycling were examined by x-ray diffraction and Rietveld analysis. The alloys mainly consisted of two or more phases from the group of Ce2Ni7-, CaCu5-, Ce5Co19-, or CeNi3-type phases. Phase abundance drastically changed with different annealing temperatures. As the annealing temperature increased, hydrogen storage capacity first increased and then decreased as indicated by the pressure-concentration isotherms. The electrochemical test showed a similar trend, resulting in a maximum discharge capacity which started at 339 mAh/g (1173 K), increased to 345 mAh/g (1198 K) and then decreased to 312 mAh/g (1273 K). The addition of Pr and Nd created a new AB3 phase with a CeNi3 structure after annealing, which contributed to better high rate dischargeability due to both a lower transferring resistance at the surface/electrolyte interface and a higher bulk diffusion coefficient.