Study on structure and electrochemical properties of the (LaGdMg)Ni3.35-xCoxAl0.15 (x=0-2.0) hydrogen storage alloys

Hydrogen storage alloys (LaGdMg)Ni3.35-xCoxAl0.15 (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 effects of partly replacing Ni by Co element in (LaGdMg)Ni3.35Al0.15 on the phase structure and electrochemical properties of (LaGdMg)Ni3.35-xCoxAl0.15 alloys were investigated. Structure analysis showed that the alloys consisted of Ce2Ni7-type (Gd2Co7-type), CaCu5-type, Pr5Co19-type, PuNi3-type phase structure. The addition of Co element obviously reduced the contents of CaCu5-type phase and increased the contents of Ce2Ni7-type phase. However, Pr5Co19-type and CaCu5-type phase obviously increased with the high content of Co. 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 then decreased. The (LaGdMg)Ni3.05Co0.3Al0.15 alloy electrode exhibited the maximum discharge capacity (392.92 mAh/g), and the (LaGdMg)Ni1.85Co1.0Al0.15 alloy electrode showed the best cyclic stability (S100=96.1%).