Microstructure and electrochemical characteristics of surface-treated Mm(Ni-Co-Al-Mn)4.76 alloys for nickel-metal hydride batteries

The microstructure and electrochemical characteristics of surface-treated Mm(Ni0.04Co0.20Al0.04Mn0.12)(4.76) alloys, (a) induction-melted and subsequently annealed alloy and (b) rapidly quenched and subsequently annealed alloy were examined to clarify the influence of the microstructure on the surface treatment effects. The surface treatment was conducted by dipping and stirring the alloys into an HCl aqueous solution of pH 1.0 at room temperature. Their initial discharge characteristics and charge-discharge cycle performance were evaluated by an electrochemical half cell and a sealed test cell. The surface structure was analyzed by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with an energy disperse X-ray spectrometer (EDX). It was found that a more homogeneous microstructure present before the surface treatment leads to an increase of the specific surface area and to an enrichment of Ni and Co metal on the surface layer of the surface-treated alloy. This leads to an enhancement of the initial electrochemical activity. (C) 1999 Elsevier Science S.A. All rights reserved.