Effect of Co substitution for Ni on the microstructure and electrochemical properties of La-R-Mg-Ni-based hydrogen storage alloys

Hydrogen storage alloys La_0.63Gd_0.2?Mg_0.17Ni_3.35?x Co _x Al_0.15 (x?=?0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0) were prepared by induction melting followed by annealing treatment in argon atmosphere. The electrochemical properties of La_0.63Gd_0.2?Mg_0.17Ni_3.35?x Co _x Al_0.15 (x?=?0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0) alloy electrodes depended on the alloy structure type. XRD patterns and EPMA showed that the alloys consisted of Ce₂Ni₇-type (Gd₂Co₇-type), CaCu₅-type, Pr₅Co₁₉-type, and PuNi₃-type phase structure. Pr₅Co₁₉-type and Ce₂Ni₇-type phase increased with the increase of Co content x. However, CaCu₅-type phase firstly decreased then increased as Co content increased. Rietveld analysis showed that the c-axis lattice parameters and cell volumes of the component phases increased with increasing Co content. The electrochemical measurements showed that as the Co content increased, the maximum discharge capacity and the cyclic stability of the annealed alloys both first increased and then decreased. The La_0.63Gd_0.2?Mg_0.17Ni_3.05Co_0.3Al_0.15 alloy electrode exhibited the maximum discharge capacity (392.92 mAh/g), and La_0.63Gd_0.2?Mg_0.17Ni_1.85Co_1.5Al_0.15 alloy electrode showed the best cyclic stability (S₁₀₀?=?96.1 %). The electrochemical kinetics studies indicate that La_0.63Gd_0.2?Mg_0.17Ni_1.85Co_1.5Al_0.15 exhibited a higher rate dischargeability (HRD₉₀₀?=?86.3 %). Electrochemical analyses showed that the control process of alloy electrode reaction is charge-transfer rate in surface film of alloy.