Electrochemical performances of cobalt-free La0.7Mg0.3Ni3.5-x(MnAl2)x (x = 0-0.20) hydrogen storage alloy electrodes

Cobalt-free AB3-type La0.7Mg0.3Ni3.5-x(MnAl2)x (x = 0-0.20) nonstoichiometric alloys were synthesized and investigated for their electrochemical hydrogen storage properties. It was found that the maximum discharge capacity of La0.7Mg0.3Ni3.5-x(MnAl2)x (x = 0-0.20) alloy electrodes decreases from 362.7 mAh/g (x = 0) to 293.9 mAh/g (x = 0.20) with increasing x. The cyclic capacity retention rate C100/Cmax of alloy electrodes initially increases to 43.7% (x = 0.10) due to the formation of Al-containing passive film, and subsequently decreases to 35.9% (x = 0.20) since the passive film is cracked. These phenomena are demonstrated by scanning electron microscopy (SEM) images of alloy electrodes after 70 charge-discharge cycles. It was also revealed that the electrochemical reaction kinetics is retarded when x < 0.10, which can be ascribed to the increase of charge-transfer resistance of the alloy electrodes due to the formation of Al-containing passive film. When x > 0.10, the electrochemical kinetics is enhanced because of the crack of the Al oxide film on the electrode surface resulting from the dissolution of Mn into the alkaline electrolyte. The optimal content of Mn and Al in La0.7Mg0.3Ni3.5-x(MnAl2)x alloys for negative electrodes in alkaline rechargeable secondary batteries is x = 0.10 in this study