Electrochemical Properties of the La[sub 0.7]Mg[sub 0.3]Ni[sub 2.65−x]Mn[sub 0.1]Co[sub 0.75]Al[sub x] (x = 0-0.5) Hydrogen Storage Alloy Electrodes

In our endeavor to improve the cyclic stability of La-Mg-Ni-Co type alloys, 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 phase and the 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% to 73.8% 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 alloys in this study is in the range from 0.2 to 0.3. © 2004 The Electrochemical Society. All rights reserved.