Electrochemical properties of the La0.7Mg0.3Ni2.65-xMn0.1Co0.75Alx (x=0-0.5) hydrogen storage alloy electrodes

In our endeavor to improve the cyclic stability of La-Mg-Ni-Co type alloys, La0.7Mg0.3Ni2.65-xMn0.1C0.75Alx (x = 0-0.5) hydrogen storage alloys were prepared and the structure and electrochemical properties of these alloys were investigated systematically. X-ray diffraction and Rietveld analyses revealed that the major phases are the (La,Mg)Ni-3 phase and the LaNi5 phase. Electrochemical studies indicate that as Ni is progressively substituted by Al, the cyclic stability of alloy electrodes is noticeably improved due to the formation of a dense oxide film on the alloy surface. The capacity retention of the alloy electrodes for 100 cycles increases from 32.0% (x = 0) to 73.8% (x = 0.3). The maximum discharge capacity decreases with increasing Al content, and the high rate dischargeability, electrochemical impedance spectra, linear polarization, Tafel polarization. and potential-step studies all indicate that the electrochemical kinetics of the alloy electrodes is deteriorated. We ascribe this phenomenon to the increase of the charge-transfer resistance of the alloy electrodes and reduction of the diffusion rate of hydrogen from interior of the bulk to the surface due to the presence of the aforesaid Al-containing oxide film. The optimal content of Al in La0.7Mg0.3Ni2.65-xMn0.1Co0.75Alx alloys in this study is in the range from 0.2 to 0.3. (C) 2004 The Electrochemical Society. All rights reserved.